Lower sound from 1st input when second input has sound
The DJ mixing desk device (mentioned by F. Bloggs) used to be called a "ducker" as it 'ducked' the music level below the voice level. If you can't find a ready-made device, you should be able to find a suitable circuit on a web search. You'll also find terms describing how quickly it ducked - too sudden doesn't sound too nice - and the release time - how long the silence has to be before the music starts to fade back up - and the release rate - how quickly it fades up once it has started.

The ducker is also a feature on many building PA systems to mute the music during paging.

https://en.wikipedia.org/wiki/Ducking may help.