Little suggestion. Maybe you should change the title... Add [SCRIPT],[POC], [ANNUNCE] or [WIP] before the name

Announce is good idea. Otherwise, most people may think it's some...one asking, if his N900 can act as a sonar. Which may result in whole array of reactions - moving along without reading is just one of them

I think It can detect direction to, if you alter the sound in channels.

So, like the way our ears work, but in reverse? (two sounds/one microphone rather than two microphones/one sound)

It would be a great idea to take some developer eh did this idea of ​​a real theremin n900

You need two 'lines' for theremin, maybe proximity sensor near front cam could be the second one? (pretty crazy)

If it's emitting clicks for sonar, they might not be very musical for a theremin.

A theremin could be fun, though.

Maybe you could use the front camera to calculate distance by the monocular depth cue apparent size of a hand: smaller as it's farther away, larger as it's closer, and whether it's at nearer the left, right, top or bottom.

Do you mean this kind of clicks?
Then we possibly may achieve it via espeak?

As a rough estimate, the maximum range should increase by factor 9 due to the better sound propagation quality of water compared to air, but the accuracy will drop by about factor 4.3 due to the higher speed of sound (and therefore smaller delay).

The details depend on signal frequency as well as medium temperature and pressure.
I assumed something in the range of 1-2kHz, 25°C and sea level.

Could range and/or accuracy be increased by attaching more powerful speakers and/or a better microphone?

One could detect movement in another room that way, too (which would certainly be a range increase) .