For playing synced audio I wanted to set-up Snapcast with Spotify Connect support. There are indeed some tutorials available for running a Snapcast server together with librespot and Home Assistant on the forums here which didn’t work for my HassOS installation, there’s a snapcast add-on which didn’t run for me (and I couldn’t get fixed) and there’s the official Spotify Connect add-on which doesn’t include Snapcast.
So I combined these things into a new add-on to run Snapcast server/client with out-of-the box support for Spotify Connect (using librespot). Also the snapweb web-interface is available. See GitHub - corneyl/hassio-addons: Home Assistant add-on repository.
I hope someone can use it for his local set-up as well! Local playback through the snapcast client is not yet tested btw, since my HA Pi doesn’t have audio output, so feel free to report if it works .