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.
Step 2: Open the Case
Disconnect the mobile's armature from the control case, and unscrew the box from the side of the crib. Remove the batteries. Undo the four triangular screws connecting the two halves of the case.
Inside you'll see two wires connecting from the control board on the front half to the battery compartment on the back. Be careful when you open the halves, and especially where you set down the back half. If you stress those wires and break one of the solder joints, you'll have to fix it yourself.
The "volume switch" (a simple two-position SPST switch with three contacts) is also mounted on the rear half of the case, with three wires (red, white, and black) leading from it to the circuit board mounted in the front half. Again, be careful not to stress these wires or you'll have a much larger project on your hands.