This is the second time that my Mac has suddenly begun playing melodies and songs without having any such program on.

The first time it happened, I restarted the computer. I'd worked on it for many hours doing editing in Final Cut Express 4 and then some music started playing although there was no program open. Anyway, the restart solved it.

Now, my brother has been playing chess on it for the last half-hour. Suddenly, the music has started playing and it seems that wherever it can access an mp3, it's playing it. I turned off the Chess voice program and it didn't solve the problem.

What should I do?

Try Force Quit [Command-Alt-Escape] to see what's actually running. If there's no app running that is likely to be playing music, you may be having some kind of RF interference issue.

If you can move your iMac to a different location, you might be able to tell if it is a problem related to where your computer is situated ie if it doesn't happen when your Mac is running in another location a mile or so away.

Also try unplugging any unessential peripherals.

Are you still under warranty or AppleCare? Apple might be able to suggest something if all else fails.

Thanks for the suggestion Colin. Will try it. I didn't think of trying the Apple equivalent of Ctrl+Alt+Delete which brings up the applications in Windows.

(It's not under warranty or Apple Care.)

Make sure you don't have Safari or some other web browser open in the background ... it may be pointing to a website that is auto-playing that unwanted musical accompaniment

Cheers
Andy

Also check your System Preferences application (not preferences inside FCP). Go to System Preferences>Accounts>Login Items. In the Login Items tab you'll see a list of applications that launch automatically when you're computer starts up. You can delete them and prevent this from happening again. :)