In a reverberant space, with speech as the source this is solidly a research problem, doubly so if there is any background noise...
You also need to specify threshold values for probability of detection and probability that target is inside the given 15cm circle.
I very much doubt that you will find code for this as an existing library, odds are you will have to write your own, and I would suggest starting with a whole pile of modelling in matlab or such before even thinking about going to a micro.
I would be thinking in terms of some filtering followed by a whole pile of autocorrelation and cross correlation, some of the public literature on passive sonar may be of interest.
Regards, Dan.