I was always lead to beleive that a virus can not be attached to an mp3. Simply because a mp3 can only be sound and a virus is data. If the mp3 file is a virus masquerading as an mp3 file, the media player will NOT play it because it does not recognise it as a coded media file.
It's another reason to keep your audio/media player up-to-date. Many will release updates from time to time to thwart off possible exploits from happening when reading the metadata in multimedia files.