OS X :: Can't Change File Associations In Leopard

I cant change file associations on my system (10.5.1) no matter what I do. For example, I had a simple wish - I wanted to wean mp3 files from iTunes and have them open in QuickTime. I was able to change the association via Get Info - Change All, but this never survived a restart. Every time I rebooted, there was iTunes again. I trashed iTunes, restarted and bingo, there was another application associated with mp3 files - not QuickTime. The system set this new association without ever consulting me, and there was nothing I could do about that.

Since then I have tried everything I could find on the web: I tried to set default associations with RCDefaultApp, I cleared all kinds of caches, I tried resetting Launch Services with Yasu, rebuilding Launch Services database with Onyx or manually in Terminal via lsregister, changing type and creator with FileInfo and SuperGetInfo. NOTHING WORKED. Nothing I configured survived more than one restart, the system always defaulted to its own mysterious preferences rather than mine. Ive spent days trying to fix this issue, with zero result. I am a recent switcher, and I must say this has been a deeply disappointing experience. Setting a file association takes about three clicks in Windows. Ive had to waste many hours of my time trying to accomplish this simple task on a Mac, and Im still getting nowhere.