[PROBLEM] Multiple devices playing through one moode device

[CallMeMike]

I've got 4 computers and a miscellany of portable devices in my office, and one good pair of speakers hooked up to my Moode Audio on a Pi Zero & miniboss.

I'd like to be able to play music on any device, and still hear the beeps, pings, and occasional videos from other devices, without having to juggle audio sources. (E.g. I'd like them all to just be mixed together). Is this possible?

I tried having multiple devices play via bluetooth to the Moode box, but the mix is clearly having a hard time time slicing (not sure if this requires the multiple cores of a Pi3 or Pi4). I've a pair of headphones that does a moderately good job of this for 2 devices, so was hopeful for Moode. 

Is there another way? Airplay? Or is this hopeless and I should drag my ancient analog mixer out of the attic?

Thanks!

(don't shoot the messenger)

1. Raspberry Pi can ONLY deal with one source at a time, I2S or LAN or WiFi or BT or USB
2. (analogue sound mixer or any analogue source) How are you going to 'inject' the analogue sound into the RPi ?
3. (if #2 solved) this solution requires drivers and integration into MoOde

Better understanding from reading about MPD, the actual Linux music player MoOde is based upon...