These things have been around for ages come in a variety of shapes and sizes. The other day I won an auction for the RoadEqualizer Box which is essentialy a sound effect keychain built into a case with a bigger speaker. My circuit did have a pitch resistor and the value was about 150k. After replacing that resistor with a photo-resistor I was able to use my opto-sequencer on it to create some awesome synthy sounds. Definatelly recommend this device to anyone who is starting out. They are cheap, under $2 except for the StarTrek one thats $10, and can be played much like opto theremins with only minor modifications. I will post samples from my mod very soon. In the mean time check your local novelty store for these babies.
Available from several places right now.
StarTrek Effects Keychain