I believe the original comment isn't about the receiver, but about the emitter -- that if you use audible-range sound or even ultrasound, the spatial resolution of the signal will be impossibly large compared to a nanobot. Each nanobot will be able to get the signal, but you won't be able to only communicate with nanobots in a specific part of the body.

This might not be a fatal objection, since you could imagine some sort of protocol with unique addresses or whatnot, but it's an objection.