how to implent watermark into mp3?

[yasuo]

Hi there,

im looking for a way to implent watermarks into audio(mp3) files.

I thought of just generating a voice speaking some code and mix it into the audio file.
it shall not disturb the audio, so i thought about stretching and/or phase-shifting it to some frequency humans cannot hear.

Would that work and wich (commandline)tools can be used to do this scriptable?
i would prefer to run the script on a LAMP-Server...

Or maybe there is another way?

greetings :-)