Jitsi, licode, janus ... there are a whole list of them, open-source. Unfortunately they conflate the MCU feature with the SDP exchange feature. Which may or may not suit your application. For instance if you want to connect clients in anything but an everyone-in-the-room-hears-everyone-else model, they don't support anything else.

Bingo. You hit the nail on the head. To efficiently do large conferences, you need to both multiplex audio and video at the media server layer and have a concept of active speakers.

You can write your app to use one of those MCUs for large conference, and accomplish the audio/video multiplexing via those MCUs. But only in a logical full-mesh configuration. So for instance 100 people listening to 6 panel members but not one another - not easy. Not unless the clients cooperate and carefully trim the streams for audience peers (fail to subscribe to them), just leaving the panel streams. It becomes complicated very fast.

Jitsi Videobridge has both of those.

Jitsi only does the audio portion. There's no support for layouts/video mixing.

"Jitsi Videobridge does not mix the video channels into a composite video stream, but only relays the received video channels to all call participants."

Joe, this is actually not true. At least not for Jitsi where we have things like last N and possibilities to have 1:m or n:m sessions ...

There's also absolutely no usage of SDP Offer/Answer in Jitsi Videobridge