After a year of using JR MC, I finally got around to figuring out how to make MC the default music player. For those skilled in Ubuntu this will not come as any news but for those with limited knowledge of Ubuntu, like myself, it may be useful.
Step 1: Open the terminal and type "cd to /usr/share/applications" and change "media center 21.desktop" to mediacenter21.desktop (get rid of the spaces). Then edit the file (sudo gedit mediacenter21.desktop) and add "%F" (no quotes) to the line Exec=/usr/bin/mediacenter21. Save and close. You will want to copy this file to the Desktop folder as well.
Step 2: Install (if you do not have it on your computer already) dconf from the Ubuntu Software Center. Open up the application after it is loaded.
Step 3. Go to com -- canonical --indicator--sound.
Step 4. Under "interested-media-players" enter mediacenter21 in the block parentheses - my entry reads ['mediacenter21', 'rhythmbox']
Step 5. Under "preferred-media-players" change the entry to ['mediacenter21']
Close the app and logout and then log back in.
Mediacenter is now the default player and shows up in the sound menu.