Quicktime X... no sound in many but not all files

After upgrading to Snow Leopard, if you have not run Disk Utility and used +Repair Disk Permissions+, you should try that now.

As a test, create a new admin user account in System Preferences Accounts pane.

http://docs.info.apple.com/article.html?path=Mac/10.5/en/8235.html

Log out and log in to the new account, where you will be use default user preferences settings for Snow Leopard and running standard background processes (unless you installed third-party software that runs at the system level). Try playing those same files using QuickTime X. Does it work properly or does the problem recur?

I've been reading some stuff online about the fact that QuickTime X currently has no built-in way to set its preferences. It is likely that QuickTime X was not 100% ready, so it was shipped with its settings locked in a default state. There are apparently ways to change its preferences using Terminal commands, or a third-party GUI

http://megabytecomp.com/apps.aspx

Perhaps the settings you need to run those AVI files are in the preferences, which are currently not available to be changed. No doubt, Apple make it available in a system update.