Incandescent streetlights give a deep hum, their AC light output is a pure 120hz sine wave. Neon signs sound different, with a squealy high frequency buzz component to their 120hz fundamental. Automobile headlights are DC, so I never tried viewing them. I put together a better viewer recently. A Seimens BPW34 PIN photodiode an opamp front end gives a bit more gain than my selenium cell.
Headphones give much better low frequency response than a speaker.
Any light source appearing in the field of view of one side will be heard as optically demodulated audio picked up by the other. Click here to see the opamp schematic. You want to become familiar with the sounds of all conventional light sources, including lamps, headlights, aircraft, fires, and if you manage to crank the gain high enough, the twinkle patterns of various stars. So far I've not encountered any mysterious lights. Vanguard Comments - this should apply to any light that can be focussed or imaged, including microscopes or telescopes.