When Joan is visiting the guitar expert and finds out how much the guitar she has in her hands is worth, she places the guitar with its neck furthest away. When the expert comes to sit on the couch, the neck is next to Joan.
The microphones that are used to detect gunshots are only designed to pick up sounds loud enough to be a gunshot, which is at least 120 dB, they are not designed to and are not capable of picking up and recording voices.