If you a QuickTime-compatible movie that suffers from bad audio quality, I think I can offer a workaround of sorts. In QuickTime Player (you'll need to have QuickTime Pro, however), select Window » Movie Properties. On the Audio Settings tab, change the db setting to +6, and the Balance to one speaker (left or right; it doesn't matter). Now instead of two speakers distributing the sound, you have one speaker playing it all. In my testing with an otherwise non-listenable audio track, this worked to make the audio understandable.
[robg adds: I don't have any clips with bad audio tracks, so I can't really confirm this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070602205313832