multiplexing control signal on speaker cable
Nutty application, but here goes ...







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Carrying DC current on speaker pair.

Figure 1 shows one way of doing this.


In normal operation audio is passed through (large) capacitors C1 and C2 to the speaker. L1 and L2 present a high impedance to audio frequencies and prevent the audio getting back or loading V+ and prevent it reaching the relay coil.
When SW1 is pressed the DC current rises (hopefully slowly enough to be inaudible) and shows up on the relay coil.
D1 freewheels when SW1 is opened and allows the current to drop gradually.


I'm not working out component values for you!