We have an infant mobile (Tiny Love's "Symphony-in-Motion" with remote) which plays annoying-after-the-Nth-repetition electronic versions of classical music fragments, at either high or low volume. Since our daughter really enjoys watching the mobile, the obvious solution to our annoyance was to install a mute switch.
Step 1: Parts and Tools
A subminiature SPST three-position switch. My lab has a drawer full of C&K 7203 switches, which is actually SPDT. For a slimmer SPST version, order the C&K 7103 (pictured) for a few bucks from Mouser, Allied, DigiKey, Jameco, or your favorite distributor. This switch has three positions -- the middle one is open-circuit, just what we want for blessed silence.
Triangular screw bits. Our mobile's case has some, really weird "security" screws with triangular recessions in the caps. I ordered a set of four bits from McMaster-Carr Industrial Supply (items 5941A11 through 5941A14).
A soldering iron, solder and flux. You will need to know how to solder already.