I've got Snow Leopard 10.6.4 completely working on the Gigabyte G31M-ES2L (have rev 1 and 2 boards).

If anyone is having Sleep problems, make sure you have the latest SleepEnabler.kext in your Extra/Extensions folder, and it seems that a 'compatible' graphics card (i.e. NVidia or ATI) may be the key to getting sleep to work. I tried to get sleep working on an ASRock mobo, as well as both Gigabyte mobos (Snow Leopard) using the onboard graphics and they would never sleep (same may be true for incompatible graphics cards).

I put a PNY NVidia GeForce 9400 GT into the rev 1 and sleep worked. I put a PNY NVidia GeForce 8400 GS into the rev 2 and sleep worked. I also tried the 9400 in the ASRock mobo - and sleep worked. I'm using the EFI String (via OSX86Tools) for the appropriate graphics card in the system's com.apple.Boot.plist (Library/Preferences/SystemConfiguration). I tried the NVEnabler kext before using the EFI String and initially it worked, but then broke after updating the SleepEnable.kext, so for me, the EFI String option has worked best for sleep and graphics. I've been using that method on my rev 1 mobo all year and never had a single problem - and sleep has always worked 100% (with updated SleepEnabler.kexts).

For audio problems, try AppleAzaliaAudio.kext (goes into System/Library/Extensions via 'Kext Helper b7'). I've tried other audio kexts but had problems with them (i.e. audio distortion after sleep), so AAA kext has worked brilliantly for me on 4 different mobos (both Leopard and Snow Leopard) over the last 18 months.