some time ago someone posted about some kind of shooting targets (gun or rifle shooting that is). if i remember correctly he used 4 microphones and a little math to calculate the coordiantes of a hit.
the target area was a little larger than a cd though (like 1x1m?) PLUS his sound waves travelled through air which makes them like 8 times slower than through the plastic of a cd.
sound travels a little faster than 300m/second in air. that's about 30cm in a milliSecond, or still 0.3mm in a microSecond. for plastics the speed usually is higher than 2000meters/second.
as mem put it: this is tricky if not impossible with your pictured setup.
good luck though.
//kuk