I've never had this problem before. I'm running xp, sp3. I import the avi file but Vegas Movie Studio Platinum 9, with c build doesn't bulid peaks. The video runs fine but no sound at all. It has sound okay in Windows Movie maker and Realplayer and WMP.
Sounds like the audio format is not one recognized by Vegas. One thing a lot of people do not understand - "avi" is actually a "container" specification and can actually contain a number of different formats for both video and audio. There can be all sorts of different things in there so while an "avi" file from one source may work fine, an "avi" file from a different source may not work at all.
Just to let you know I worked round the problem by using Feemake Video Converter to convert the video to an MP3 file, then imported that into the audio track. The sound matched perfectly with the video track.