Plex works, and Synology Audio Station works too.

In general, I like Plex, but I've been encountering occasional problems with its Chromecast implementation. Sometimes Plex on my iPhone and/or Plex on my housemates' Android devices won't see the Chromecast. And when they do finally find the Chromecast, they lose connection to it sometimes, or do weird things such as crash out when you just intended to press pause.

All the other apps on our phones which can Chromecast are fine, so I can only assume that the Plex code for the Chromecast isn't fully baked.

I still use Plex when I can, though.