The recorded sound is the interference pattern between multiple sound sources. They need to install sophisticated directional-detecting equipment and extremely high-frequency recording equipment. I suspect that the large signals have frequencies that are well-above the spectrum of human hearing, and what is currently recorded is just the interference pattern between multiple high-frequency signals.
Realize that high-end high-fidelity audio equipment reproduces sound way above the spectrum that humans can hear. The best of us can hear up around 20 kilo-Hertz, but high-end audio systems can produce 100 kilo-Hertz. This is because most percussion instruments and some non-percussion instruments in Orchestras produce interference patterns that are in the human-perceptible audio range, and it’s the interference patterns produced when the super high-frequency sound is reproduced that we hear.
We weld plastic with ultrasound. What do you think those high frequencies will do to neural tissue at high power? Look at the spectrum provided under the “Ultrasound” section here: http://www.learneasy.info/MDME/projects/A1A2_project/robot_sensors/robot_sensors.htm