Choosing an input signal based on whether a driver signal is floating
A multiplexer will require extra circuitry around it and will be a bit of a hassle.
Maybe go for a more brute force but robust solution: a relay.
It's just a mechanical contact so there's no concern about signal voltage levels, which a multiplexer solution would have to contend with.
Relays are about $2 and come in different coil voltages: 5V, 9V, 12V are common.
For example you could use a 9V relay and 9V battery: apply 9V across the coil and the switch closes.
Relay parameters to be aware of:
-you want a &quot;non-latching&quot; relay
-a single-pole single-throw (SPST) relay is fine, but using half of a DPDT is fine too
-switch contacts are often labelled (N.O. = normally open, and N.C. = normally closed).
&quot;Normally&quot; in this context is the switch state when coil is not energized.
So in the schematic below the MIC_OUT net is connected to the &quot;normally open&quot; pin of the relay.
Note the relay will make a small acoustic click when toggled. If anybody complains tell them it's a feature: it lets them know they pressed the button.



simulate this circuit &ndash; Schematic created using CircuitLab