I’ve wanted to try transmitting audio using light for a long time, and this afternoon I finally tried it. To modulate the beam I shifted the MP3 player output using a MCP602 op amp. 3 V +/- 0.3 V seemed quite ideal. I generated the 3V reference using a zener diode (the current limiting resistor doubled as the R4 of the aforementioned level shifter circuit). This level shifter is in the left side of the picture.
The receiver circuit (top right) consisted of a phototransistor with a few passive components for setting the current through the transistor and a high pass filter. I amplified the signal using a TBA820M amplifier chip (in the middle) and used an 8 ohm speaker for output.