you update the BIOS on your NF7-S because BIOS updates usually sort sound problems out. Otherwise it's best to write to abit and tell them your system spec and the exact problem or you can RMA the board as it shouldn't happen.

I know quite a few people who have issues with their sound. Personally i don't have a problem because i have a set of DSP-500's so i don't use the onboard sound atm yet i've never had a problem with it anyway.