Is this problem solved now for you after the update to .757?
It hasn't for me. I still get the annoying broken/disturbed notification tone many times even after a clean install of the FW .757.
I am on stock ROM, non rooted. I really do want to know which app is playing rogue, if it is an app. I thought after the few updates to the walkman app, it may solve the issue, but it didn't.