Came across this on the r/selfhosted community. Still very much in the alpha stages, but it’s already got a Docker image you can try out for yourself, or try out the demo server.
Tried it earlier today, couldn’t get the voice/video chat to work right away on my self-hosted setup but the real-time chat was very snappy. Looks promising.
Drop-in voice channels are a requirement for a discord alternative, Matrix does not have them. AFAIK you still need to call an entire channel to start a voice chat.