I just went through a bunch WMV files and the same thing happened. I also had it happen specifically after waiting to make sure the audio was conformed so that's definitely not the issue.
I did think of one part I left out. About 90% of the time that it happens, the audio works fine at first, but then after I fastword by dragging the playback marker it stops. It would seem to indicate that maybe it's still doing some type of loading. Sometimes after this happens if I rewind back to the beginning of the clip, the audio plays. Other times, after I rewind back to the beginning, it still doesn't play.
Perhaps I'm experiencing more than one issue? That could explain why it seems random. If I had two different problems that are causing audio to drop out, it would be much harder to see a pattern since the two separate problems could contradict each other.