Just an idea,
I've had a similar problem, I was using DV files from different cameras with audio at 32 and 46mhz (or whatever the standards/units are) so I had the properties set to the higher level, I think it was because Premiere was "conforming" the audio clip in the backround, then after it had conformed the clip, which took a little while, I could play it back fine, with sound.
look out for a "conforming audio" message in the status bar at the bottom right.
That might be the cause of it if the problem goes away after a bit. Check your project audio properties are correct and it might not happen.
Panasonic SD9, Panasonic NVGS-75, Canon MV600i