The Amazon music app does not play their HD or Ultra HD music. It will only play their normal mp3 music. The Roku will play back FLAC, but the Amazon Music app (unless they've updated it this past week) doesn't serve up FLAC to the Roku. Similarly the Tidal app wont play their MQA files, nor will it play their CD quality files.
Interestingly, however, Tidal over Plex will play back their CD quality files, but Tidal over Plex, whether in Roku plex app, or PC, or any other Plex app, will not play Tidal masters either.
I just bought the latest Roku 2020 model 4800X with the hopes that the new audio passthrough option would allow Amazon Music HD to perform hi-res playback to my AVR, but of course true to Roku’s form (astoundingly and continuing; short-sightedness) it does not support this. Maybe it is Amazon’s fault, if so then I owe Roku engineers an apology. In either case I wish someone would fix this issue. It would be great to be able to stream hi-res audio from Amazon HD to my AVR. I am sure I speak for many users on this issue.
I too am disappointed with Roku vis a vis HD & UHD music. The Amazon Music (AM) rep. Becky D on the AM discussion board says her Samsung smart TV with AM installed will show UHD badges. Roku won't, as far as I know.
But there is more to actually playing UHD on an AVR than just finding a source of HiRes music. I'm sure you know this, but many AVR's just a few years old do not have the ability to playback UHD digital files without decreasing the bitrate & sampling depth. I asked Yamaha about my AVR and tech replied that my AVR did not have a DAC that was capable of converting HiRes music. The way to get my AVR to play HiRes at 24/96 is to hook up a HiRes DAC to the AVR, bypassing the Yamaha internal DAC. I did this with an LG V30 phone. I have noticed that the new Yamaha models do have DAC's that are capable.
Well I usually stream to my Anthem MRX-720 from Amazon Music HD via DTS Play-Fi using an iphone or ipad, so I can get hi-res playback, that is bit perfect, but I miss the visual portion, you know the album art and lyrics being displayed on my TV. That is why I was hoping Roku would solve this problem by allowing the audio to actually “pass-through” and allow my AVR to decode the signal, of course the MRX-720 has very high level DACs built into it. I have a Denon AVR in my living room that uses HEOS, which I am impressed with, which of course provides the visual content display for Amazon Music HD and claims to playback in hi-res. However some people on various forums and articles that I have read recently claim that HEOS actually does not play back in true bit perfect form and compresses the audio information. HEOS sounds pretty good, but I am not sure I can confirm the actual bit rate being played by the Denon AVR.
Getting back on topic here, it would just be nice, and seems so simple, for the Roku to actually “pass-through” the audio signal to my AVR without altering it. I suspect the Amazon Music HD app on Roku is the true culprit, it just doesn’t have the audio signal for HD music playback. I remember when when Roku/Amazon took down the first Amazon Music app because it was trash, and then several long months went by before the new app came back. That was a long wait.
Oh and BTW the Apple TV 4K does not pass through audio bit streams either from Music apps to my AVR, nor does any Of Amazon products. So I am left with DTS Play-fi to stream hi-res audio from my online music services, but again the visual content is missing.
I have Amazon Music Ultra HD and HD streaming from my Fire Cube into my Yamaha RX-V683 AVR. Check your Yamaha setup manual in the specs page for the audio capability. My AVR runs from PCM 2- 8 channels at a max of 24/ 192.
I haven't tried in a month or two, since I returned my 2020 Roku Ultra in favor of an nVidia Shield TV Pro, but at that time, the Roku would pass through hi-res audio (which prior models would not), but the Amazon Music app would only play back mp3 audio. I was able to play back hi-res audio over plex and tidal (well, Tidal's CD quality), but not Amazon. This is Amazon's issue, but you wont just have a bad experience on Roku, the Android and AndroidTV apps are garbage too, though, they'll play back at higher quality, it's just that Amazon Music HD is just generally buggy and has a bad UI. I'm sure it works great on Amazon's first party devices, or at least better, but likely nothing spectacular even there.
The only way to play back Atmos music via their service (which yes, it supports), is through their Echo Studio which can then play back to the fireTV cube or fire stick, but you have to have the Echo Studio first... And if you are expecting them to make it any better... well... at least the interface for Prime Video doesn't suck QUITE as much as it used to. (Still sucks though).
I don't subscribe to Amazon Music HD. I am a free Amazon Music user. To play Amazon Music on Roku, I always try AudFree Amazon Music Converter to convert Amazon Music to MP3 and customize its audio parameters to meet my Roku settings.