You may also want to check your Control Panels->File Exchange preferences, aka Internet Explorer's "File Helpers".

This control panel is responsible for mapping file extensions (like .mp3, for instance) to the "corresponding" Mac OS file type and creator code information.

All files in Mac OS 9 and prior, whether a document or application, have "type" and "creator code" attributes. These attributes consist of four letters (or numbers) that help identify which documents belong to which application and vice versa. For example QuickTime Player has type "APPL" (Application) and creator code "TVOD." A quicktime mp3 file will have a creator of "TVOD" and type "Mp3 ". The "TVOD" part is what identifies that it was created by QuickTime Player. When you double-click the file, it's opened by the application that "created it." "TVOD" is unique--no other Mac OS application has this creator code--in fact you normally register your app's CC with Apple when you create a new app. This code is also responsible for determining what icon is associated w/ the file.

When you download a file off the internet or using LimeWire, those files do not originally contain type and creator code information. All the file has is the 3 letter file extension, ".xxx" The File Exchange control panel's "PC Exchange" tab is the point where that "file extension to creator-code and file-type" mapping takes place.

If you indeed don't have RealPlayer, and wish to open the file in Quicktime Player, use the "File Translation" tab to set that preference. Click the "Add..." button, locate the .mp3 file that's associated w/ Real Player, and choose to open it w/ Quictime Player.

You may also want to try rebuilding your desktop file by holding down the Command+option keys right after all of your extensions load but before you see the Finder/Desktop and icons.

Hope this helps...