I noticed that a lot of people new to OS X have a problem setting up multichannel out/in. So, below is a cut and paste from another thread where we discussed this. This is all prior art, not the dark secret of the Templars but, hopefully it'll help someone out. For your pleasure... I give you : The Surround Sound How -To.
/*Now with 95% Recycled Information and Colour Plates*/

To set up surround sound on OSX requires the use Audio MIDI Setup.app that should be in your utilities folder. In it, hit command-shift-a to open up aggregate device editor (or selected it from the menu). You need to create a new aggregate device then in the structure box assign all the outs to it. It doesn't matter if they are head phones etc, we will fix it up later. The annoying thing is that sometimes an in socket may have become an out.

Once you have an aggregate device, in the main Audio Midi Setup screen select it in the "Properties For" bob, hit configure speakers. Select Multichannel and 5.1 surround. Strictly speaking it can be any set up you have going; 7.1, 8.1 etc. That's the beauty of this app - got 5.1 but want 7.1; buy a USB dongle and add its stereo out to your new device. The next bit is fairly self explanatory; if you click on the speaker button it will send some white noise to it - you assign which channel is being used with the bobs. Hopefully you end up with all your speakers working and assigned to correct channels. Click apply and done. Now Set the aggregate device as Default Output and voilá; multichannel sound. After setting up multichannel, don't forget to set up the stereo sound as well. Otherwise you might find that music comes out of pretty odd speakers.

In an ideal universe.... YMMV. It works well with some equipment - less so with other. The keyboard volume control often ceases to work (volume control in apps, eg. VLC, iTunes works fine). AFAIK, this works more smoothly with USB (eg HK's iSub) and digital outs than it does with analogue.

In the Aggregate device editor you can drag the sound outs up and down, this means that, for example, you can have FL&FR on streams 1&2 -I do not know if it has any impact beyond aesthetics. Make sure all channels are set to same bit & sample rates. You can make ad-hoc multichannel using a couple of $5 USB sound dongles, make sure you have the clock set to build-in audio to avoid channels going out of sync. Oh, you can rename your device by clicking on it.

The most powerful function of this tool is the ability to create an aggregate INPUT device. (e.g. join up line-in and mic-in and you have a 4 channel multi track recorder) You can then select your new aggregate device as a sound source in Logic or GarageBand. (Works w/ protools & Cubase too). Want to add more channels - you are only limited by the number of your firewire and USB ports. (Well.... I made a monster 24 channel beast on my old powerbook; recording 24 tracks chocked the laptop HD promptly (or the CPU, it was a 550MHz G4)).

This has to be one of the least documented, most powerful tools that we inherited from CoreAudio - why Apple keeps so quiet about it always puzzled me. The interface looks like it came out of an undergrad Java 101 assignment, it's almost as if a programmer left it there as an after thought.

As an example, here is how it worked out on my GA P35-DS3P rev.2 :

The plugs that work for me are: Line Out (aka "Built-in Speaker"), Sub/C Out (aka "Line Output") and front headphone socket (that's the header on your mobo, aka "Headphone"). The last one can be a bit iffy; the pins on the mobo can be connected as either AC97 or HDA audio. It makes little difference for os X (afaik), but can make things tricky in windows -if your case has an old school AC97 header - like many (most?) on the market still do.

To make this setup work in Vista you will need to go to Gigabyte's website and download the realtek driver and utility. This will enable hot plug detection. After you install the driver, unplug and replug your speakers one by one; you will be asked what speaker you plugged in and can assign RL & RR to the headphone socket. (You can use any other socket for you headphones... or, I find it easier to use the desktop remote for my speaker system - if you have one)

The problem occurs if your headphones are connected to the AC97 standard, hot-plug won't work and you will have no way to remap headphones to rear speakers. (Well.... I'm sure there is a way - I'm just not enough into windows to work it out.) In that case, you have to either; disable rear speakers in config, keep switching the plugs around or, wire up a Y splitter so you can have them plugged into two sockets at once.



P.S
http://www.apple.com/pro/techniques/aggregateaudio/
Oh, and the device for logic has to have at least one out... At least it used to have to.