5Volt has published a design for a simple light to sound converter:
The circuit is very simple and is based and pretty well known building blocks. The whole device can be built on a small piece of perfboard and the output can feed a pair of head phones or a small speaker.
PC speakers could replace the amplifier stage but at the cost of portability. The circuit would easily fit into an Altoids tin box.
The use is immediate, just put the battery on and play with the gain and volume while pointing not directly to a light bulb : you should hear a strong buzzing noise (the 50/60 Hz mains), then try any TV (CRT or LCD), any remote control or flame.
Looks like a fun project. Details at the site.
Here’s a video of the sound converter in action: