Circuit to cut one audio input into another
I know that they sell analog mux ICs that can take in two signals and output one or the other. The fun part is actually knowing when to turn one or the other on right?
The music channel is the &quot;main&quot; channel, as in it is usually the one on. You could use a peak detector on the radio signal and feed that into a comparator to know when to toggle the mux channel. This way would allow you to tweak how long it takes for the radio to switch on (to avoid random noise and such), time to switch off after noise has stopped (to avoid switching to music mid-sentence), and the level at which you toggle. This is a two edged sword though, as you have to tweak it to make it work.
If you can, try recording some waveforms from the radio as it goes into your earpiece. You can import that into a spice program and try it out for yourself.