In preparation for Plai 2015 festival (happens once a year in our city, Timisoara) we made a series of experiments at our makerspace Plan Zero, which included some very cool laser projections!

After lots of failed attempts in which we mounted keychain lasers on speakers made from disposable plates and other funny ideas, we found the lucky combination: two toy motors vibrate two tiny mirrors. One controls the vertical, the other the horizontal. The motors vibrate because they are connected to the output of a quite powerful amplifier, each on a separate channel. Because the sound signal oscillates above and below 0 volts, the motors do not rotate, instead they vibrate at the same frequency as the audio signal. The sound goes from the computer to the amplifier, then to the motors.
Music looks like this:

Not very interesting because the frequencies are about equal on each channel, so that projection forms ellipses.
This is how two different signals on each channel (left&right) look, one being a sine wave at 100 Hz, the other a sine wave which goes from 80 to 200 Hz (or something like that, can't remember exactly):

The proportions between frequencies creates Lissajous figures.
We also tried viewing the waveforms (sine and saw) by sending them to a single little motor, the other being spun as slowly as possible by hooking it to the battery through a potentiometer:

As shown in the videos, because we used the small toy motors, the projected image "jumps" at certain frequencies. This is one of the limitations of such a simple device.