In the larger of the two blobs (eggs? orbs?) is a small commercial amplifier that I stripped down and retrofitted. You can see it runs off of a 9-volt battery. The stock volume control sucks, so I installed a more sensitive volume control in the other half of the pair. In the smaller blob is a simple homemade oscillator, which also runs on 9 volts. I can control the frequency, or pitch, of the tone quite carefully. In this version I've used a photo-sensitive resistor in the circuit, which you can see as a tiny little eye on the smaller blob. A photoresistor is exactly what it sounds like: a light-sensitive resistor. Changing the resistance in the oscillator circuit causes a change in the frequency, so here the resistor acts as a sort of motion detector; as a person gets closer to the small blob and cuts off some of the light (even a tiny amount), the frequency of the tone will change and a new sound will come out of the big blob. I like to think of it as involuntary user interaction with an initial element of surprise. I will have 5 or 6 of these guys playing their own tones very quietly, though I haven't decided if they will all play the same tone or something else.
As soon as I figure it out I will post a quick snippet of the sounds themselves. Until then consider it a surprise.