If you modify the Chromecast profile Tautilli reports Plex is transcoding from 'ALAC Stereo to FLAC Stereo', which I assume must be bit-perfect.Chromecast is not bit-perfect for me since I have my library in ALAC. It always transcodes.
Plex Media Server\Resources\Profiles\Chromecast.xml
Code:
<TranscodeTargets>
...
<MusicProfile container="mkv" codec="flac" />
...
</TranscodeTargets>