Network protocol over audio
This question was asked right before I joined the site, so I didn't see it until just now, when it was brought to my attention for review.

You can use any of various "audio watermark" techniques to embed a low-speed (tens of bps) data signal into any sort of audible "cover" signal. This means that what a human hears does not necessarily need to be related to the actual content of the communication (but it can be).

As for simplicity and low cost, I once built a prototype watermark decoder that was intended for embedding in interactive toys, which is an extremely cost-constrained market. It was based on a 6502 microcontroller clone along with a simple opamp-based analog signal chain for gain and filtering. The demo system, using a low-cost electret microphone, could reliably pick out watermarks across a conference room from a laptop computer driving cheap speakers.

Your question as it currently stands is really not a good fit for EE or the SE format in general. The question about existing protocols you could use is probably better suited to the DSP site. But to get a meaningful answer, you need to be more specific about the kinds and amounts of information you actually want to exchange among the various units of your project.