@Deanor30 HDMIAudio.kext.zip   @all users   trial_AppleALC_Lilu_Optiplex780_755.zip
Please try (this is trial AppleALC1.3.0 and Lilu1.2.5): Here the LineIn's work on my Optiplex780 now with simple external Mic.
For Optiplex755 also changed, please try.   Optiplex780 / 760:
LayoutID 11 or LayoutID 13
The codec in AppleALC is broken for 780/760 since 10.13.4. Rollback AppleHDA from 10.13.3 is necessary.
only Mojave Bootflags -lilubetaall -alcbeta   Optiplex755:
LayoutID 28
only Mojave Bootflags -lilubetaall -alcbeta