Kerry Wong has built a great Laser Oscilloscope using two Speakers. Of course this is not something that can be used to measure any real voltages or waveforms with any accuracy, for that you can use any used $50 scope. What this one does is produce a great visual display. You would be able to record separate right and left audio tracks to produce any light show you like.
Via: Dangerous Prototypes
“A rod (must be made of none magnetic material) was glued onto the cone of the speaker using epoxy at an angle. So when the speaker vocal coil is energized, the vertical movement of the speaker cone translates into the vertical movement of the rod. The other end of the rod was glued onto one axis (one edge of the mirror). Another set of identical actuator mechanism was used to control the other axis. A long screw was used as the support point of the mirror. The length of the screw was chosen so that the mirror sits on the screw tip and can rotate about freely. The picture on the right above shows this configuration.”