I downloaded MusicMatch and installed it with no problem. It asks during the installation process if you want to use it with different sound files. You can say "no."

Try this:
Open MusicMatch
Click on Options, then Settings
In the General tab, Click on the little boxes by the types of files to open with MusicMatch to take the check marks out of them (I don't see RealAudio in there. It's possible you'll have to re-install RealPlayer if the steps above don't solve the problem.)