Ive just installed Ubuntu on my desktop pc getting rid of windows completely, and everything except the audio is working. Ive got a creative x-fi sound card but ive just removed it as it doesn't seem to be supported yet and I thought it could be causing conflicts. Im currently attempting to use onboard audio (my motherboards the asus a8n-sli premium) but it still doesnt seem to be working. In the sound preferences it lists two soundcards, the nvidia ck804 and the MPU-401 UART, then in the volume control the devices are: Nvidia ck804 (alsa mixer) and realtek alc850 rev 0 (oss mixer). If anyone could help it would be greatly appreciated, thanks.

Go to Realtek's site - http://www.realtek.com.tw - currently it's broken! What a luck?
Download linux drivers for ALC850. Make sure you have linux-headers package installed as well as gcc compiler packages (along with libc6-dev).
Run the Realtek driver package - it's pretty straigh forward. Open terminal; change directory to where you've downloaded it to and run it.
After reboot it should load the new module/driver for ALC850.
I've had the same problem.

Go to 'System' -> 'Preferences' -> 'Sound' first and check the "default sound card" is your sound card, if not click it and try to select your sound card.

If that fails then install the driver. You may need to do "sudo apt-get install build-essential" before you try to install the driver, as you'll probably have to compile it first.