I just successfully got the audio working on my Asus M4A785TD-M EVO motherboard after a substantial amount of effort. I run Debian 2.6.26-2-amd64. To the best of my knowledge, the problem appears to be an old version of the alsa module that comes with the distro that doesn't support the audio card/chipset on this mobo. Here are the steps I used with the caveat that of course these may be unique to some extent to this setup. You might need to run the following as root using sudo or su. I'm assuming you have gcc and the kernel sources installed correctly.
1. Make sure you have the packages installed...
aptitude install alsa-base
2. Download alsa-driver-1.0.23 from http://www.alsa-project.org/main/index.php/Main_Page
into whatever directory you're working in.
tar jxvf alsa-driver-1.0.23.tar.bz2
3. Run alsaconf.
4. Run alsamixer and unmute the master channel!
That's it! If that doesn't get the thing playing, you could try passing specific arguments to the module using:
modprobe snd_hda_intel model=6stack
But I'm not sure this last step is necessary so long as you compiled and installed the 1.0.23 module successfully.
I will update if I have any more insights - I'd be interested in hearing the experience of others with this frustrating problem.