A teacher came in today with a Quicktime file that had sound but no audio track. How so, do you say? The video and audio were “muxed” (short for “multiplexed’) into a single track. While I cannot comment on the advantages of muxing, iMovie couldn’t import the file, even though a different copy of iMovie first created the file (by exporting at full quality).
I enjoy Googling for unique words, because you get results so quickly. Google led us to a free utility called MPEG Streamclip, which not only can separate the video and audio tracks of a muxed file, it also purports to convert many other formats, including flv, avi, YouTube (via URL), iPhone, and QuickTime transport files. This looks like required equipment for film teachers and others who use digital video.