I decided to find out how I could go about changing which media player opened up when I pressed the Gnome "launch music player" keyboard shortcut. I found that there was a bug filed
because there is no way to change it in Ubuntu.
There is actually a very simple way to get around this bug. This requires either uninstall rhythmbox or renaming /usr/bin/rhythmbox and then creating /usr/bin/rhythmbox as a symbolic link to the executable of the music player you want to launch. I didn't want rhythmbox so I used apt-get to remove it, but I'm sure many users would rather rename the executable for rhythmbox.
Here is what you could do to make my media launch key quodlibet:
sudo mv /usr/bin/rhythmbox /usr/bin/rhythmbox_backup
sudo ln -s /usr/bin/quodlibet /usr/bin/rhythmbox