Well, Jenny - RealPlayer is for playing RealAudio recordings (which are kind of like tape recordings), and doesn't have anything to do with MIDI files. If you have a sound card, your computer will also probably have a MIDI driver installed. The MIDI function of your sound card is a little synthesizer. MIDI files are like player piano rolls, recordings of the keystrokes that make the MIDI operate. Is that easier to understand?
If you have a sound card installed and a MIDI driver properly configured, you should be able to click on the name of any MIDI file on your hard drive, and it should begin playing. When I've had sound problems, I've usually been able to fix them by reinstalling the software that came with my sound card.
Hope that helps.
-Joe Offer-