Modem-M64 can only be used as a pair.
A Modem M64 can only be in sync and communicate with one other modem at any given time.
It is possible to operate several modems in the same area by using different channels so they won't interfere with each other.
It is also possible to disconnect a modem and connect to another modem by switching the channel. All Modem-M64 will automatically sync up with another modem which is on the same channel and within its acoustic range of 200 meters.
The protocol can be found at the following link.
An overview of the available channels can be found at the following link.