The latest quicktime version is 6.3

No, it wouldn't play automatically on Windows Media player or real player. For a player to play a type of audio format, it has to be set to do so, and said setting is not automatic. Try using either of those aps to open the avi by manually opening it via the program. That could work. If it does, set the program that does that to always open avis by going into the preferences.

If none of that works, please post ur OS ur specs and more details of the problem.