Step 1: Get a cheap keyboard with a volume knob / use your busted one :)
However, just take all the screws out, and remove the controller board, which on the cheap keyboards is normally one board that has the 3 LEDs on it for the lock keys, and it'll also have a jumper cable headed over to a daughter board that has the volume knob on it. (I already stole the volume knob in this photo)
Step 2: Assemble
Super glue the daughter board to the top and then put the wheel on it, and route the wires so that they don't get ruined when you close up the box.
You're done :>