Watermarking sound, reading through iPhone

I want to add a few bytes of data to a sound file (for example a song). The sound file will be transmitted via radio to a received who uses for example the iPhone microphone to pick up the sound, and an application will show the original bytes of data. Preferably it should not be hearable for humans.

What is such technology called? Are there any applications that can do this? Libraries/apps that can be used on iPhone?

I've done some research, and it seems the way to go is:

• Use low audio frequencies.
• Spread the "bits" around randomly - do not use a pattern as it will be picked up by the listener. "White noise" is a good clue. The random pattern is known by the sender and receiver.
• Use Fourier transform to pick up frequency and amplitude
• Clean up input data.
• Use checksum/redundancy-algorithms to compensate for loss.

I'm writing a prototype and am having a bit difficulty in picking up the right frequency as if has a ~4 Hz offset (100 Hz becomes 96.x Hz when played and picked up by the microphone).

This is not the answer, but I hope it helps.