I've noticed that both my Chromecast v2 and Ultra are happy to direct play multi-channel audio encoded using either Dolby Digital (AC3) or Dolby Digital Plus (EAC3). Anything else (Dolby True HD, Atmos, DTS, DTS-MA, DTS-X) gets transcoded to Dolby Digital at 384 kbit/s. I'm very impressed with the way Emby handles this - it's even smart enough to transcode EAC3 to AC3 when I turn off the AV receiver (and it's just the TV doing the audio decoding). I'm less impressed that it needs to do this at all for the Chromecast Ultra: why do Google think it's OK to market it as 4K HRD capable and yet only support these two codecs? It's particularly bad when DTS gets transcoded to AC3 (one lossy early 90s format to another).
Dolby claim that DD+ is up to twice as efficient as old DD and Netflix are now streaming in DD+ at 640 kbit/s ("the point at which additional quality is imperceivable"). I decided to experiment using Emby's version of ffmpeg on my Synology NAS (a DS918+ running a Celeron J3455). DTS to AC3 at 384k vs. DTS to EAC3 at 640k. I had to set avoid_negative_ts to its default value of "auto" for the EAC3 transcode - Chromecast won't direct play EAC3 streams with negative timestamps. I was concerned that the EAC3 conversion might require more CPU time, but if anything it's faster. The conversion logs are attached.
I won't claim to have the best equipment or ears (MP3 at 320k is fine by me), but I can hear the difference. The Mad Men opening theme (test file I used) sounds better to me in the EAC3 version, particularly the cymbals. What do the Emby developers think about transcoding to EAC3 at 640k when it's supported by the playback device? An easy win for better audio quality, or just another wild idea from a clueless audiophile?