I’m not too sure, I didn’t look terribly deeply on that front since that’s not something my group uses often, and its difficult to test on my own. That said, I believe Matrix does have some level of voice chat support, though AFAIK its more similar to Skype’s calls than Discord’s drop-in/drop-out voice channels. No idea about XMPP though.

I belive just Matrix and XMPP.

From my digging on alternatives the main contenders are (in no particular order)

• Stoat
  • Essentially 1:1 on discords format
  • UK based, so its future there is uncertain
  • Infrastructure is lacking, was crippled by the initial influx after discord’s announcement.
  • Missing some minor UI and UX features, feels unpolished.
• Spacebar
  • Reverse engineered discord
  • Greatest potential, IMO, but as of a few days ago all it has is potential.
  • lacks significant client development, relying on an external client named Fermi, which feels quite amateurish.
• XMPP
  • Highly mature, looks very promising, but lacks any kind of guild/nested channel grouping support which makes it unsuitable for my group, so I didn’t look too deep at it.
• Matrix
  • IMO the most likely discord successor.
  • Minor functional hiccups, that vary from client to client
  • Of the clients I tested, Cinny is the most discord like, but I hear commet is closer.
  • Nested spaces provides the minimum format equivalence.
• Fluxer
  • Slightly sus vibes
  • Lacks self hosting instructions
  • Media is non-permanent, which is I guess fair to keep infra costs down, but its unsuitable for my groups media usage habits.
  • Looks promising, but I’ve not actually tried it given the lack of self hosting instructions.

One thing that’s wormed its way onto the to do list that haunts the back of my mind, is I’d like to see if I could abuse the matrix or XMPP protocols to get some of the nicer discord-like features lime invite links, server side channel ordering, and space membership over channel membership. But that’s unlikely to happen any time soon.

EDIT: Forgot Fluxer. Added.