Step 1:

First u want to get an old pair of earbuds you don't use.

Step 2:

Next get an old speaker .remember it has to be small enough to fit inside the monster lid.

Step 3:

Now strip the earbud wire with a knife, razor blade or, wire snips. Then sand of a a centimeter off the end of the colored wire and solder it to the speaker. Next u can put electrical tape over the wires or a shrink boot with a lighter.

Step 4:

Find a speaker grill and cut it the size of the speaker and drill out the top of the cap and put in the speaker grill u can hot glue it into the can lid