I have the same motherboard (Asus M2N-E)...

I (finally) got my audio to work using the Azalia Audio drivers off the Zephyroth Leopard AMD 10.5.2 Rev.2 DVD. I made sure to ONLY check the Azalia Audio option --during my like 4th install--under audio drivers and my audio actually works now.

There is a AzaliaAudio.pkg package here http://osx86.wikidot.com/utilities-resources that I assume is the same one from the DVD, it might work for you.

I still can't get my video to fully work, they only way I can do a "normal" boot (without using the -x boot flag) is to delete GeForce.kext.

You might want to try this: http://nvinject.free.fr/512Mb.html

Good luck, you'll need it