I just decided to turn loose Win8 RP on "bare metal" here as opposed to a VM. I didn't even bother to mess with video under a VM, but with the bare metal install, I've run into some trouble.
At first, media player didn't seem to play anything, including .wmv files. Messing around, I unchecked the "Direct X acceleration" box and that allowed it to play .wmv files. However, it still won't play .mp4 files -- no video, just sound, and Media Player will hang trying to exit.
It did this with a clean install with only the Windows supplied video driver. My video card has been obsoleted by AMD now (HD 4200 chipset) and I couldn't use the new Windows 8 preview driver package, but I did try intalling the AMD 12.4 driver in Win7 compatibility mode. That worked fine it seems, but it still won't play .mp4 files. Turning the Direct X acceleration back on still kills .wmv playback as well.
Any ideas? Win7, both x64 and 32-bit played .mp4 files fine right out of the box on this machine.