Muliplexing enables several signals to be sent over the same channel simultaneously.
In the top diagram, the Multiplexer rotary switch samples each channel in turn, and connects it to the link.
The Demultiplexer switch connects each listener in turn, to the link. As long as the two switches are rotated in synchronism, Listener 1 will only hear Talker 1, etc.
The minimum sample rate need only be twice the highest frequency of a talker signal, according to Nyquist. In practice, electronic switches are used.
A synchronising signal is required to keep talkers and listeners in step.