Now that you've got your toy, get out your screwdriver and open the thing up. Be careful to keep track of all of the screws! A digital camera is also really helpful--take pictures at various steps in the process, like before removing something major, so that you can retrace your steps backward if necessary.
The best suggestion I can give is to simply experiment: play around with different values of resistors, different connections in the circuit, with capacitors, diodes or inductors, with putting yourself in place of a resistor, etc. A good place to start is to just connect different points in the circuit together with wire and see what happens. And if you end up causing all sound to cease, you might have crashed the processor; just remove the batteries (to reboot the processor) and try again.
