Open the keytbl.ini file in the Road Runner folder. Put the following line in it:
Tapping F9 will then mute the music, and tapping again will unmute the music.
RR does not look at if a key is held down or not, if you hold a key, it will just repeat like when typing somewhere.
Optionally, you could use
120,"PLAY" which would pause when tapped, and resume when tapped again.