Thanks to:
THe KiNG, stickpin, Kabyl, karaakeha1, MadTux, Taruga, OcciJano and everyone (will be listed if I remember) who published kexts and information.

Knowledge & How to:
[HOW TO] Patch AppleHDA - Knowledge Base by THe KiNG
How to patch AppleHDA.kext from 10.5.2 and above, Ii is a little bit hard, but is fun. by MadTux
[How To] Pinconfig for Linux users by Master Chief
Audio Device Technologies for Windows from Microsoft
Pin Configuration Guidelines for High Definition Audio Devices from Microsoft
Legacy AppleHDA for BOOT-132 by stickpin
[How TO] 5.1 Surround Sound by consolation
[How to] Have sound on all of your speakers by macgirl
How to get 5.1 audio out of AAC-surround-encoded audio

DSDTs:
DSDT: trick retail drivers by changing "device-id" (e.g USB) by zhell
ACPI (DSDT) Patcher for BIOS and AML by koalala
DSDT Patcher by fassl
DSDT fixes for Gigabyte boards by iSoprano
DSDT - Vanilla Speedstep - Remove _cst errors for MB without C-states by FormerlyKnownAs

Installing a patched AppleHDA.kext, legacy kext, and HDAEnabler.kext to whare?
1) A vanilla AppleHDA.kext or a patched AppleHDA.kext must be installed into /System/Library/Extensions only.
2) Any legacy kext(s) and HDAEnabler.kext can be installed into /Extra/Extensions, /Extra/Extensions.mkext, or /System/Library/Extensions.
3) LegacyHDA.kext contains only info.plist so it has to work with AppleHDA.kext.

4) HDAEnabler.kext is not required if you are using DSDT.aml with HDEF injection.
5) layout-id in DSDT or HDAEnabler.kext must be matched to LayoutID in info.plists of LegacyHDA.kext or AppleHDA.kext.
6) To avoid popping sound is setting model to be MacPro.

How to enabler sound: there are three ways as I've known. Use only one that works for you.
1) Using HDAEnabler.kext.
2) Using DSDT with HDEF injection.
3) Using EFI string.

How to get sound working:
1) vanilla AppleHDA.kext + LegacyHDA.kext + HDAEnabler.kext (or DSDT w/ HDEF injection) for ALC889a/ALC885
2) vanilla AppleHDA.kext + LegacyHDA.kext + DSDT w/ HDEF injections and HDAU (HDMI audio) injection for ALC889a/ALC885 plus nVidia GT220*
3) bin patched AppleHDA.kext for ALC### only + LegacyHDA.kext for ALC### + HDAEnabler.kext (or DSDT w/ HDEF injection)
4) bin patched AppleHDA.kext for ALC### plus nVidia GT240* + LegacyHDA.kext for ALC### + DSDT w/ HDEF & HDAU injections
5) bin patched AppleHDA.kext & edited info.plists for ALC### + HDAEnabler.kext (or DSDT w/ HDEF injection)
6) bin patched AppleHDA.kext & edited info.plists for ALC### plus nVidia GT240* + DSDT w/ HDEF & HDAU injections
7) bin patched AppleHDA.kext & edited info.plists for ALC### only plus nVidia GT220* + DSDT w/ HDEF & HDAU injections
8) ALC### disabled plus nVidia GT220*: vanilla AppleHDA.kext + DSDT w/ HDAU injection
9) ALC### disabled plus nVidia GT240*: bin pathced AppleHDA.kext for nVidia GT240 only + DSDT w/ HDAU injection

*for HDMI audio, AppleHDA.kext v1.8.4fc3 or higher is required.

HDMI:
There are two methods to get audio via an HDMI cable.
1). For a graphic card that has a S/PDIF connector , read Post #759.
2). For a graphic card that has a built-in audio chip, see Post #760.


[b]Software:
Kext Helper b7
Kext Utility for SL


For OS X 10.6

10.6.4
LagacyHDA.kext for ALC889a is working fine. Don't need to patch binary.

Others:
 AppleHDA_1.8.7f1_ALC662_LayoutID_662.zip ( 1.26MB ) Number of downloads: 190

 AppleHDA_1.8.7f1_ALC888_LayoutID_888.zip ( 1.28MB ) Number of downloads: 768



10.6.3
 How_to_patch_AppleHDA.zip ( 1.07MB ) Number of downloads: 1927

For using a new AppleHDA.kext (1.8.4fc3 or later), ALC662, ALC883, ALC888B (or ALC887), ALC888 and ALC889 will not be able to use LegacyHDA.kext without patching AppleHDA binary. This is DIY. You can make your sound card working. Gigabyte mainboards that have ALC889a can use LegacyHDA.kext with the latest AppleHDA.kext without issues.

Patching AppleHDA binary and editing all needed info.plists are recommended instead of using LegacyHDA.kext.

This is an example of patched AppleHDA.kext. If you are able to patch AppleHDA binary and edit required info.plists, it should be like this example.
Since you are using this one, you don't need legacy/dummy kext.
 AppleHDA_1.8.4fc3_ALC888_LayoutID_888.zip ( 1.26MB ) Number of downloads: 866


ALC662: based on my 945GCM-S2L, LayoutID = 662.
 AppleHDA_1.8.4fc3for_ALC662.zip ( 1.23MB ) Number of downloads: 292


Vanilla
 AppleHDA_1.8.4fc3.zip ( 1.36MB ) Number of downloads: 502


10.6.2
Fixing for 10.6.2 by removing BuiltInHDA and including HDAEnabler.kext
 ALC889a_LayoutID_885.zip ( 73.85K ) Number of downloads: 3326


10.6 & 10.6.1 & 10.5.8
For Gigabyte (E)P35 & (E)P45 and may be compatible to other Gigabyte motherboards on how to make DSDT.
Making DSDT.aml for your PC is better.
There are LegacyHDA.kext for ALC889a and ALC888. 10.5.8 & Snow*
 How_to_patch_DSDT_with_LAN_and_HDEF_functions_including_LegacyHDA.zip ( 552.38K ) Number of downloads: 10846

Also see more info about HDEF @post #392 for ALC889a
Also see more info about HDEF @post #498 for ALC888

Series of LegacyHDA.kext for Gigabyte ALC888 and ALC889a 10.5.8 & Snow*
Series of LegacyHDA.zip or  Series_of_LegacyHDA.zip ( 414.67K ) Number of downloads: 4206


HDAEnabler for someone doesn't want to edit DSDT with HDEF.
HDAEnabler for Snow.zip

*For using my LegacyHDA.kext with AppleHDA.kext v1.7.9a4 (10.6.2) or later, please remove BuiltInHDA key from info.plist of LegacyHDA.kext, see an example.

by using TextEdit:

or using Property List Editor to remove BuiltInHDA key, see a picture:
 legacyhda.jpg ( 68.12K ) Number of downloads: 3507


Some other Gigabyte ALC88#(x):
ALC662 = ALC662..........found on 945GCM-S2L
ALC883 = ALC883..........found on G31M-ES2L
ALC888B = ALC887 *** found on G41M-ES2L

For testing Gigabyte P55 and P55A users: You may need to update your BIOS. Link to BIOS update
Gigabyte_ALC888b_P55.zip
Gigabyte_ALC889_P55.zip not for ALC889a
For GA-P55A, ALC889: please go to Post #677 by WintonAce for more info and LegacyHDA.kext

###############################################
You may use patched AppleHDA.kext of OS X 10.5 to be run on OS X 10.6 at 32bit mode.
###############################################

For OS X 10.5 for Gigabyte ALC889A, old stuff.

Before download edited AppleHDA.kext, please read followings:
 readme.pdf ( 17.93K ) Number of downloads: 5573
---> will tell you which kext you want.
 errors.pdf ( 31.29K ) Number of downloads: 1746
---> explains some errors of my previous edited AppleHDA.kexts.
 CodecVerbs.zip ( 4.74K ) Number of downloads: 2084
---> CodecVerbs, 889-00.rtf can be used for Gigabyte ALC889A & ALC888.

Requirement: OS X 10.5 or higher
for ALC889A layout-id = 12 ; 0x0000000c ; 0x0c, 0x00, 0x00, 0x00
from 10.5  1.4.0a22.zip ( 409.6K ) Number of downloads: 1768


Requirement: OS X 10.5.4 or higher
for ALC889A layout-id = 889 ; 0x00000379 ; 0x79, 0x03, 0x00, 0x00
 1.5.7a24_3outs3ins_digital_HDA_headphone.zip ( 534.19K ) Number of downloads: 1140

 1.5.7a24_3outs3ins_digital_regular_headphone.zip ( 535.83K ) Number of downloads: 391

 1.5.7a24_4outs3ins_analog_HDA_headphone.zip ( 534.21K ) Number of downloads: 628

 1.5.7a24_4outs3ins_analog_regular_headphone.zip ( 535.45K ) Number of downloads: 407


Requirement: OS X 10.5.6 or higher
for ALC889A layout-id = 889 ; 0x00000379 ; 0x79, 0x03, 0x00, 0x00
 n1_1.6.2a37_3outs3ins_digital_HDA_headphone.zip ( 603.48K ) Number of downloads: 644

 n2_1.6.2a37_3outs3ins_digital_regular_headphone.zip ( 604.48K ) Number of downloads: 321

 n3_1.6.2a37_4outs3ins_analog_HDA_headphone.zip ( 603.21K ) Number of downloads: 653

 n4_1.6.2a37_4outs3ins_analog_regular_headphone.zip ( 604.25K ) Number of downloads: 370


Requirement: OS X 10.5.6 or higher
for ALC889A layout-id = 889 ; 0x00000379 ; 0x79, 0x03, 0x00, 0x00
These following files have no front mic that causes of "Sound assertion "0 == fMikeyDriver" failed in "/SourceCache/AppleHDA/AppleHDA-162.1.37/AppleHDA/AppleHDADriver.cpp" at line 1063 goto handler."
 N1_1.6.2a37_3outs2ins_digital_HDA_headphone.zip ( 603.32K ) Number of downloads: 538

 N2_1.6.2a37_3outs2ins_digital_regular_headphone.zip ( 604.33K ) Number of downloads: 213

 N3_1.6.2a37_4outs2ins_analog_HDA_headphone.zip ( 603.05K ) Number of downloads: 363

 N4_1.6.2a37_4outs2ins_analog_regular_headphone.zip ( 604.09K ) Number of downloads: 281

------------------------------------------------------------------
Credit to stickpin for making legacy kexts.
working with AppleHDA.kext version 1.5.7a24, 1.6.2a37
for ALC889A layout-id = 889 ; 0x00000379 ; 0x79, 0x03, 0x00, 0x00
 Legacy_3outs3ins_digital_HDA_headphone.zip ( 21.74K ) Number of downloads: 718

 Legacy_3outs3ins_digital_regualr_headphone.zip ( 21.96K ) Number of downloads: 303

 Legacy_4outs3ins_analog_HDA_headphone.zip ( 21.67K ) Number of downloads: 535

 Legacy_4outs3ins_analog_regualr_headphone.zip ( 21.91K ) Number of downloads: 356


No front mic available.
 series_of_legacy_no_front_mic.zip ( 95.22K ) Number of downloads: 474


Requirement: OS X 10.5.7 or higher
1.6.8a3 AppleHDA, front mic is not available. layout-id = 885 ; 0x00000375 ; 0x75, 0x03, 0x00, 0x00
 HDAEnabler_for_1057.zip ( 11.88K ) Number of downloads: 1756
This HDAEnabler.kext should be used with 1.6.8a3 AppleHDA.kext.
 L1_3outs2ins_digital_HDA_headphone.zip ( 630.14K ) Number of downloads: 759

 L2_3outs2ins_digital_regular_headphone.zip ( 631.01K ) Number of downloads: 342

 L3_4outs2ins_analog_HDA_headphone.zip ( 629.92K ) Number of downloads: 671

 L4_4outs2ins_analog_regular_headphone.zip ( 630.8K ) Number of downloads: 458

 Series_LegacyHDA_1057.zip ( 109.58K ) Number of downloads: 1239

---------------------------------------------------------------------
vanilla AppleHDA.kext
 Original_AppleHDA.kext_1.5.7a24.zip ( 521.78K ) Number of downloads: 444

 Original_AppleHDA.kext_1.6.2a37.zip ( 641.3K ) Number of downloads: 1135

================================
Previous edited AppleHDA.kexts
1)  ALC889A_V1.zip ( 1.01MB ) Number of downloads: 993

2)  ALC889A_V2.zip ( 1.01MB ) Number of downloads: 1216
removed all "SignalProcessing" key from PathMapRef.
3)  ALC889A_1056.zip ( 746.25K ) Number of downloads: 660

4)  ALC889A_1056_fix.zip ( 747.3K ) Number of downloads: 2619
Front mic fixed.
5)  AppleHDA_10.5.6.zip ( 549.78K ) Number of downloads: 1302

---------------------------------------------------------
AppleHDA.kext for ALC888 (Gigabyte), tested on GA-EP45-UD3LR
1.5.7a24 888.zip
1.6.2a37 888.zip
---------------------------------------------------------
DIY
Edit AppleHDA.kext/Contents/Resources/English.lproj/Localizable.strings, see picture below for an example:
 local_strings.jpg ( 265.72K ) Number of downloads: 3641

and results
 local_in.jpg ( 67.4K ) Number of downloads: 2778
 local_out.jpg ( 59.01K ) Number of downloads: 1820

(got from AlainTox714's thread)