https://mashable.com/article/signal-end-to-end-encrypted-gro...
Jitsi has supposed e2ee videoconferencing (on Chrome - they used some chrome-specific API for processing) for (I believe) at least a year or two.
Element has native E2EE for 1-to-1 calls, but uses Jitsi for group calls. Native E2EE group calls actually also exist, called Element Call (https://element.io/blog/introducing-native-matrix-voip-with-...) but they're yet to be integrated into Element and specced into Matrix, I believe.
I don't believe this could be true, and the linked article doesn't have the word "mix" anywhere. I imagine that there is no mixing happening until after decryption on the client device. Of course this means that every audio source goes to each recipient discretely, which means more bandwidth, but audio (especially near-silent moments therein) is lightweight enough for reasonably sized groups. Obviously this same n^2 scaling issue happens with the video anyway which is never mixed.
More info is available at my blog post about it: https://signal.org/blog/how-to-build-encrypted-group-calls/
1. When the mic is soft muted, information is getting sent to the conference provider which could leak information about private matters.
2. When the mic is not muted all information is definitely going to the conference provider in a way they can decrypt so they can mix it.
That is to say that when using most conference software, you have already granted then access to contents of the meeting.
If you can't trust them to not miss use information they get when the mic is off, then you also can't trust them when the mic is on.