The first thing that I would check would be the Export settings on that offending group of computers. This ARTICLE goes into more detail, on what to look for. My first guess would be that the users on those computers made changes to the Export Settings in the Auido area. If everything IS set correctly, then we can head in another direction with troubleshooting.
The first thing I'd ask is how do you know the audio isn't there? Are you exporting two files (V and A) and the .wav is missing? Is it a muxed file and sound just doesn't play?