When Real Player installs, it makes itself the default player for mp3 as well as .avi (which it can't play on the Mac) and a bunch of other file types you may never see in a lifetime. If you install RP after QT or iTunes, it will override those settings.

Some Wintel types actually use RealPlayer for mp3's, so there is no error when you download one of these files.

Go to the File Exchange control panel and make sure iTunes or QuickTime is the only app assigned to play mp3's. Delete all Real Player stuff for mp3 (and .avi while you're at it).

Or if this is too techy for you, get iTunes Associator at versiontracker.com. Drag the file to Associator's icon. If you don't even have RP installed, this would be the easiest solution, since RP has no settings in your File Exchange control panel.