I have spent days trying to get audio to work on new Biostar A780L3C system, using Realtek HD drivers. It all started when my XP Home system complained that the setup program on the motherboard CD with all the drivers was "not a valid Win32 application."
Using Realtek HD audio drivers I downloaded from Realtek and the motherboard maker's site, I got "audio device on high definition bus"
as the device name in Device Manager. Whoops. This was clearly a lousy install. No sound, of course.
It's important to be sure the Microsoft UAA HD audio driver is installed. It's there by default, but there was a suggestion to disable it. Don't do that.
I solved this by downloading a VIA HD audio driver from Biostar's site. Apparently, it''s generic enough to work. It installed instantly and I had sound.