Theremin

touch, click, or focus and press an arrow key

A theremin is played without touching it: you move your hands through the field around two antennas, and the sound follows. The vertical rod sets the pitch, the loop sets the volume.

Here the whole screen is that field. Drag or tap anywhere — left to right bends the pitch, bottom to top swells the volume. Both are continuous, so there is no note to miss. Focus the surface and hold an arrow key to play it from the keyboard.

On a phone, the hand holding it is your second hand. While a note is sounding, roll the device to bend the pitch and tip it away from you to damp the volume — both measured from however you were holding it when the note began, so there is no pose you have to find first.