Sounds like it’s experimental at the moment:

Voice/Video when?

Currently there is experimental voice/video WebRTC support in Spacebar. UDP connections are not currently supported.

This is a very difficult feature to get working, especially given that we must implement it the exact same way as Discord.com for client compatibility, so if you find any bugs please open an issue in Spacebar server.

We would also be incredibly thankful for any assistance.

docs.spacebar.chat/faq/

So might not be ready for primetime. Voice is pretty critical to how I often use Discord. Video less so, but still important.