Support
Theremin is made by one person. If something does not work, or you have an idea, send an email and I will get back to you as soon as I can.
Things that are answered already
"Nothing happens when I hold up my hand." Theremin needs the camera. If you said no when it asked, turn it back on in Settings ▸ Privacy & Security ▸ Camera ▸ Theremin. Hold one finger up in front of the front camera, in reasonable light, with your hand fully in view.
"How do I play?" Hold one finger up in front of the front camera. Raise it for higher notes, lower it for lower ones, and move it sideways to play louder or softer. One raised finger is all it takes; an open hand plays too. In Duet Mode each hand is its own instrument.
"How do I play quieter, or stop?" Move your hand sideways for volume. To stop, close your hand into a fist: the sound goes quiet without you taking your hand out of view. Open it again, or raise a finger, and it plays on. You can turn this off in the settings under Fist to Mute.
"Is the camera recording me?" No. The Privacy Policy explains exactly what happens to the picture: it is looked at on your device, one frame at a time, and thrown away.