I don't know
I actually don't think it will work.
To your old approach:
did you try println() ?
What happens when you clap with your hands left from the screen and right from the screen?
Are both mics absolutely parallel in their println() (are we sure it's stereo?)
It could be a difference in volume.
Or it could be a difference in time: It reaches the one mic first, then the other... but probably too close.
Do the mics work at all?
Do you receive a "middle" as output via text()?
Do you have any change at println() when you clap?
Because if not, this line
- in = minim.getLineIn(Minim.STEREO, 512);
might be wrong....