It's not as unlikely as you think. I just ran into the same exact thing. But I don't think it's the WMV.
I'm using footage from a Canon 5D Mark III shot at 60fps, then interpreted to 23.976 and dropped in a 23.976 timeline. I've also adjusted the volume on some of these clips, but not down to -∞. Since the error is "...AudioRender/AudioPrefetch.cpp-99" it makes sense that this is an audio related issue. I have also noticed that in the timeline, my yellow levels line and waveforms disappear when this error comes up.
So far I've been able to get around this bug by copying the clips into a new timeline, but this eventually comes back again.
The one question I have, is are you editing from an external hard drive? I've been editing at work off an external, then backing it up and working off internal drives when I get home. My external drive has been disconnecting itself lately, but it's never been around or at the same time as this error. Thought I'd still check with others though.
I'm edinting with an internal SATA drive, backing up to USB drives.
Okay. Still no luck over here. Had to duplicate my timeline again.
The next thing I'm noticing is that this seems to happen when my drives spin down and go into hibernation/sleep mode. When I switch back to Premiere and the drives spin up again, shortly after I get that error. This might be coincidence, but I thought I'd pass it along. I'm going to try turning off the "sleep when not in use" option for my editing/working hard drives to see if that makes a difference.
For Andrew I'd recommend getting the PowerPoint exported at the proper frame rate so Interpret Footage isn't needed.
For Kris I recommend removing the audio track of the clip, which is not normally needed at all when slowing down from 60 to 24.
Unfortunately Jim, powerpoint only allows you to export a project as video at 30 fps. (Powerpoint 2010, windows)
As for the audio prefetch error, to make a nested sequence show an audio waveform, you use the Clip menu and select Audio Options > Render and Replace.
The prefetch error in some cases is connected to a file with an interpreted frame rate, so could you use the Render and Replace option to fix this? You will be making a new audio file at your interpreted frame rate so hopefully the problem will not occur.
I created a new timeline, matched sequence settings, copied the regions including the problematic frame/section, and it worked fine.