Automatic Audio Selector

[SinclairZX]

Hello,
Does anyone know of a way (or a prebuilt device) that will allow me to have 2 audio input devices and 1 output with an automatic switch between the two. For example:
On Input 1 i have a radio which is the default throughput
On Input 2 i have a microphone - when the microphone provides output the switch automatically selects Input 2 as the throughput - then diverts to Input 1 when switched off.
Hope that makes sense.. Thanks