What are you using for audio?
(or do you want suggestions on that too)
Probably an ultrasonic sensor such as the ping))) sensor or one of the maxbotics ones would be best.
We can't advise any more until we know what hardware you intend to use or exactly what you want to do and what your budget is if you would like suggestions on what hardware to use.
It's a relatively easy thing to do though with certain hardware.
Mowcius