westfw:
Actually, it's a common Radio-control hack to use a motor as an audio transducer. By feeding in a signal at audio frequencies that doesn't actually move the motor, you can vibrate the armature and/or windings enough to generate a clearly audiable tone.
Hmm, I knew that there are ways to vibrate an electric motor's armature without causing it to rotate, but I didn't know a generic DC motor could generate a sufficiently loud sound this way (at least not without possibly damaging the motor). Nor did I realize that people would do such a thing on purpose. 
I've heard "music" made by steppers or old floppy disk drive motors but to the best of my knowledge those examples always involved rotating motors.
In any case, I didn't mean to imply that 500 Hz was below the range of human hearing. I was trying to make a general statement that to get the exact PWM as a tone there needs to be a proper audio transducer. Apparently a non-rotating DC motor can be such a transducer.