Is there an AirPlay mixer? 8 AirPlay Inputs – 1 AirPlay output?

I have been working with a friend on some video production that will end up on the YT soon. Nothing very interesting.

Now we are using a Black Magic gadget from BlackMagic Mini Pro. It has 4 HDMI inputs, some additional audio inputs and you can to some extent mix them, switch, etc. As far as I know you cannot merge.

Given all sorts of different devices that we have used as inputs, getting to HDMI was not always easy and even not compatible.

I started thinking that it would be cool to have a program and/or hardware that would take multiple different Airplay (audio and/or video), treat them the same as HDMI inputs and allow mixing of the inputs.

Seems to me it could all be done in software as an application. and perhaps there are already a lot of them out there and I am just ignorant of them.

Of course having both HDMI and Airplay would be even better.

In thinking about this I figured as a messy prototype 4 Raspberry PIs converting Airplay to HDMI and then into Mini Pro.

Of 4 Mac laptops, or 4 iPhones.

I would love to hear input from others.