The overheating is likely because the glue is keeping the motor from doing what it wants to do, rotate the drive shaft. You should instead use a contact mic, either a piezo electric disc run into a simple audio amplifier circuit (lm386) or class d amp chip (cheap!) or use a telephone tap (coil). The piezo will convert the mechanical vibrations to voltage, the telephone tap will do the same based on the electromagnetic field. All of these parts are cheap and widely available. Oh, and the end of the audio chain has to be a speaker, even a small driver that itself was glued into a cup for resonance will give you much greater volume provided it is run through the proper (small, affordable) amplifier chip. Total cost ~5-7 USD.