I'm having the same problem on my new ECS (cheap crapola) 755-A2 motherboard. It has the Realtek ALC655 chip on it and I'm running Suse10 with a basically fresh install (x86-64). This chip seems to have some problems with certain linux distros, but there are a bunch of reports of it working. I've seen some people claim it works if you just revert to OSS instead of ALSA. That did not work for me. Anyway I noticed Sun uses this chip in their new 2100 series servers, so that makes me feel a lot better that this chip will be supported. I'm guessing theres just some peculiar problem rihgt now.
Anyone find a fix? Please post if you have any suggestions.