In unbalanced audio connections; one of the two signal conductors is typically at "ground" voltage potential. Because electrical signals can be both in the form of a voltage and a current; it is possible for a current to flow though a conductor that is at ground potential. For this reason; a coaxial cable's shield conductor can act as both a shield and a signal return.
It is also possible to use cable normally used for balanced connections to increase noise rejection of long unbalanced cables. This is sometimes referred to as a "quasi-balanced" connection. By using the "low" conductor of twisted-pair cable as the signal return; the shield can be disconnected at the receiving end to increase noise rejection. This approach effectively “decouples” the signal return conductor at the receiving device’s input from the noise currents that may flow through the shield; even if they are connected together at the transmitting end.
For more information see adapter cable.