Have you rebuilt your desktop recently? Restart with the Option key held down until you get asked whether you wish to rebuild. Choose yes.

If that doesn't work then time you got a decent mp3 player like Audion (click on link) So get it for free whilst you still can. It plays all formats except wmv & rm.

By the way if something won't open by double-clicking then drag & drop it or use your control panel to set up the default app to open it.