This is exactly what I'm looking for. In my case I don't need any fancy fade. Just cut the other line when a signal is detected in the other. Honestly been looking for something like this for days and can't find anything, no DIY or commercial that would be in the moderate price range.

Would think this is quite common accessary when people want to listen to music, but still want to monitor something like a CB radio, or need to monitor coms. My use case would be monitoring aviation frequency while still being able to get alerts about air traffic and music from my phone.

Mod: link to old thread.
https://forum.allaboutcircuits.com/threads/2-source-priority-mixer.163709/post-1438367

 

Maybe you could use a KA2284 voltage level indicator - either an assembled module which is fairly inexpensive, or start with the bare IC. I believe it can work as an audio level meter where a row of five LEDs progessively light up as volume increases. Pick the LED output you want to use and with a relay (using a transistor to turn on the relay) you could switch channels. A bit abrupt, no fade, but you say that you are okay with that

 

An effective scheme would be to use a circuit similar to a "volume compressor", but with the sample input taken from the signal to be monitored while the control would be applied to the signal being muted.
So now a question for the TS: How likely are you to understand a detailed circuit showing the implementation of that function?
Also, would the music need to be just reduced, or completely switched off? That affects the applied control arrangement.

 

You made the example of a CB radio while listening to music. I understand you have a different use in mind, but I'll stick with your example for the moment: You're listening to music while the CB radio is squelched. When the audio rises above a preset level, set by the squelch knob, the audio will come through. What you're saying is you want a signal to interrupt one for the other. IF you go with a similar approach to the squelch you may find that you're getting a mix of both audio sources, each one individually but punctuated by the amplitude of the modulated audio from the CB (again with your example).

I'm thinking you may want some sort of time delay whereby when you get the secondary signal you want it to cut in and interrupt the primary audio signal; but one that also keeps the primary signal cut for a short period of time. Now, just guessing at what that timing may be, but you may want to hold the primary cut and the secondary active for half of a second to possibly two seconds - or more. That way you're not hearing "Yo--e th----ly one I---ve" while at the same time hearing "--ig----323 -he--y clear to land". IF you incorporate the time period mentioned, the music would be interrupted while you hear "Flight 7323 heavy clear to land".

 

The common scheme for the better squelch systems is that when the squelch opens the level to open it is reduced. In addition, many current squelch systems trigger on the carrier level, not the audio level. And a number of them trigger on a constant pilot tone. So there are a number of schemes already in existence to solve that issue quite well.

For that exact application, if you can find it, or at least the service information, back several years when Chrysler corp offered a CB/Radio package, it had exactly that function in the CB radio module. The CB module was not very impressive, and needed an external amplifier for receive, and so not many of them were saved to build CB radios. But if that circuit information is still around it will do exactly what you ask for.
And if anybody has that information, I have a stack of those modules far back in an attic. They were free at the end of a Dayton Ham swap many years back.