still sounds like a driver issue to me. You can go
here and download the appropriate driver for your Operating system. I use this exact same mainboard as you do and have no problems with this driver and my C-media onboard sound(when I use it.......weekend LAN parties...I have an external PCI GTXP at home). I use the Windows XP version as thats the OS I'm running on. If this does not fix the situation see if its a cable issue.......cables can and do go bad especially near connector points(I'm talking about your audio cables from your speakers. If you determine it to be not related to the driver or cables then I would contact Soyo about the issue and see what they have to say about it.
Edit: On that same link above I just noticed a bit further down the page there is a link to a .zip file with an older version of the C-media driver if the newest doesnt work.....give it a try also.