Where can I download a sound/audio driver for P4M266A-8235 VIA?  

I am using Windows 7 Home Premium and I need a driver for P4M266A-8235 VIA (Chipset Vendor)

Either go to the manufacterers website or try download.com
Have you done all the windows updates ?

Seeing as Speccy and not likely much of any other system info tool will show the actual manufacturer of the motherboard likely due to using generic drivers due to its age), look at the motherboard and locate who actually made it. The chipset used was used by most if not all motherboard manufacturers as the time. The chipset itself did include Via audio,

VIA P4M266 - VIA Technologies, Inc.

but not all motherboard manufacturers would have used it, and may have opted for one of the other onboard audio solutions from the time (most if not all AC'97). Once you know who made the motherboard then visit their site and get the last XP driver for it, as there likely won't be anything for Windows 7 (or Vista for that matter) for it due to its age.