I downloaded a movie from zml and I have video, but no audio....however when I play it on my pc the audio plays fine. Just so you know, I erased the movie and downloaded it a 2nd time and still no audio and I am using windows media player (I downloaded 2 other movies using windows and the playback works fine).
Also, when you download movies to quick time, what version are you using (I have version 7 however it asks me to download pro, at $29.99 I might add, in order to playback movies for my bb).
You should not need Quick Time pro for what you are doing -- I think that's a marketing gimmick. I don't recall the lastest version of Quick Time, but you might need to update to the latest version.
It could be that the avi file in question needs to be recoded so it fit's within the paramater's. I know I tried loading in a video file that was larger than 320x240 and I was able to hear it but couldn't see it.
Roxio in Media Manager will convert videos to the correct format for the BlackBerry. Worked great for me with some videos I have in .avi format. The instructions are here: