Thefreezingmoon
Keep in mind that in order to get access to Atmos-encoded content from Amazon music an Amazon Music Unlimited subscription is required.
However, even though the Streaming Stick 4K (3820) and every other RokuOS model/device released in the last decade or so is more than capable of passing through DD+/Atmos (EAC3/JOC) used by streaming services, neither Apple nor Amazon have enabled/configured their respective music apps to allow/use DD+/Atmos on the RokuOS platform.
This is almost entirely a non-technically-based decision due to the respective companies policies on bundling/including/limiting DD+/Atmos content access.
You'll need an Amazon FireTV device (e.g. FireTV Stick 4K Max Gen 2) to get access to HD/Spatial (e.g. Atmos) audio for Amazon Music (has to be manually enabled in the Amazon Music app's settings) and a recent WebOS (LG TV) or tvOS (e.g. Apple TV 4K) for access to Atmos on Apple Music.
Otherwise you can always contact Amazon/Apple and ask/insist/demand that they enable their respective RokuOS music apps to receive/stream DD+/Atmos encoded content, but dont expect a response, let alone a policy change.