It means that the audio was encoded VBR (ariable bit rate), simply that it is bigger some places than others. If you want to edit this vido you will need to make the bitrate constant. Easiest way to do that is to go to audio- full processing mode, then file- save wav. No use that wav as the audio in further editing.
the standard release of VirtualDub simply does not handle variable bit rate mp3s correctly. if you save the audio stream without re-encoding, it will be out of synch. so you will need to re-encode the audio to a constant bit rate stream before you save the AVI... or you can use a version of VirtualDub that deals with these streams correctly. for instance, NanDub is well-suited for such files.