Since tvOS 18 update Apple TV 4K and Apple TV HD output only stereo despite 5.1 selected

Have you tried any other software changes such as enabling "trial fix" updates in Software Updates? (There are posting requirements for Communities that I simply forgot about when submitting my previous reply.)

That's the case for me, too, for whichever of my two ATV's I choose not to use with my Marantz NR1510 receiver. I have 1 Marantz receiver and 2 other multi channel audio systems. I believe Apple can fix it and I hope they will. But this is a little more complex than the previous similar issues that have bitten me. Not only does the precise nature of the hardware involved seem to determine success or failure, but it also depends on which streaming apps are used. I hope Apple doesn't simply leave it up to app developers to try to comply with the current schema. To me that would be detrimental to Apple's bottom line.

But failing to eliminate such an insidious problem from an initial major software release such as 18.0 will also have that effect, I suspect.

That entire comment was a little too long to be allowed in the comment field at Apple TV Feedback, so I removed the last sentence.