Changing Stream from Live input to a MP3 playback

[kryptolix]

Hello,
I have darkice streams running in our church. I used Rocket Broadcaster on a Laptop, but want to use a headless setup with a RPi4 and a Behringer U-Phoria UMC202HD instead. When there is no actual service we run recordings from our services in the past. On the Broadcaster program I have a small mixer, where I have live input and playback on two faders + master fader. I just mute and unmute the sources as needed while the listener doesn't have to do something.
I thought about doing the mp3 playback via a loopback device. This has not been tested yet, but I will do try tomorrow. The live input stream works as intended and was already tested multiple times on our services. While I am able to make a separate config file and reboot the stream with the needed one, the stream stops for the client and has to be reactivated.
I already thought about doing the mute / unmute procedure with alsamixer in the same fashion like in Rocket Broadcaster, I still have the problem, that the live input and playback are on different (virtual) devices in the audioconfig. Do you have ideas to solve that?
Something like let the loopback device and hwplug send their streams to a different loopback, which gets used from darkice?
Thanks in advance!