I'm surprised I have never heard of the Helix streaming server before. However, a look at the supported formats makes me think matroska is not supported. This was the absolute goal of this project: stream in the matroska format. The reason is that most Anime and other subtitled videos are distributed in matroska, because it can mux SubStation Alpha subtitles. I wanted to be able to live stream Anime with my friends without anyone having to download it.
It lets people jump in mid-stream, however syncing clients is quite troublesome, since anyone can just pause their stream. It cannot really be enforced server-side to display the exact frame at the exact time. Maybe it can be combined with syncplay[0].
Glad to see pulseaudio-support is on the roadmap (shouldn't be too hard really). On Linux I have never had a proper ALSA recording with ffmpeg, due to ALSA "xruns", i.e. bad buffers. Pulse may be using a bulldozer to drive in a nail, but it doesn't yield this problem.