It's likely that you'll have no issues streaming FLAC files from Plex, but it's unclear that it can do so bit pefectly in
all situations with the Chromecast Audio. That's the problem with using software which is heavily biased towards providing video media - audio file serving can be inflexible and unlikely to have the same careful consideration provided by media server software designed to just handle audio files.
One such audio dedicated UPnP media server is the excellent, well supported, free (donations welcome)
MinimServer, as mentioned by
@Antiquarian. You'll not to get any nasty surprises with audio file streaming using MinimServer!
Audirvana's UPnP/DLNA support actually consists of a built-in UPnP/DLNA controller as well as a media server. So if you are going to use MinimServer instead of Audirvana with the Chromecast Audio (via the BubbleUPnP Server), you are also going to need a UPnP/DLNA controller, as MinimServer does not come with a UPnP/DLNA controller.
Another thing to consider is that there are some UPnP/DLNA controller apps that also support Chromecast devices, so they would allow you to stream from MinimServer
without needing the BubbleUPnP Server's created DLNA renderer for Chromecast devices, eg, the
mconnect Player iOS app, with the free
mconnect Player Lite version available for testing. The
mconnect Player HD version, as mentioned by
@Antiquarian, has been designed to be used in landscape mode, so ideal for use with the iPad. The 'HD' in its name is about its fancy 'HD' landscape display & absolutely nothing to do with audio streaming quality.
An alternative type of app to use to stream from UPnP/DLNA media servers like MinimServer would be an OpenHome streamer controller, eg, the free
Linn Kazoo app. This would require you to still use the BubbleUPnP Server, but you'd need to create an OpenHome renderer for the Chromecast Audio, instead of the DLNA renderer you created for it previously.
BTW,
@Jamie 's suggestion of using the free
Logitech Media Server with the Chromecast bridge / CastBridge 3rd party LMS plugin is definitely worth trying as an alternative. The bonus with the CastBridge plugin is that it has a Flow option, which gets Chromecast devices to play gaplessly.
LMS actually comes with a simple built-in controller accessed via any web browser. The Material Skin 3rd party LMS plugin provides very decent display pages for LMS's web browser controller.