[Issue]: No 5.1 sound with combination FireTV Moonlight and Sunshine host

[drsno]

Describe the bug

I got a new Fire TV Stick 4k max 2nd gen, and I can't get the 5.1 surround sound working. The receiver keeps reporting stereo sound.

I tested hours with all different sound settings in Sunshine host and FireTV Moonlight, to no avail. The strange thing is that other combinations work fine:
Gamestream + FireTV Moonlight: OK, 5.1 sound
Sunshine + LG OS Moonlight: OK, 5.1 sound
Gamestream + LG OS Moonlight: OK, 5.1 sound
Sunshine + FireTV Moonlight, Not OK, 2.0 sound

So looking at things separately, it works fine. The technical setup works fine, because I can get 5.1 sound. Both when plugging the FireTV Stick in the receiver and in my LG TV. Sunshine seems to work fine, because it can stream 5.1 to LG Moonlight. FireTV Moonlight seems to work fine, because it can stream 5.1 from Gamestream.
However, the combination does not work.

One thing of note: it did work exactly once, with all the same settings. I was playing with HDR settings in the stream, and in the Moonlight host selection screen I suddenly saw that the host was not available. After a few seconds, it was available again, and I resumed the stream. At that point I saw some popup status messages (it was quick, but at least it said something about 'handshake'). I normally don't see these popup status messages. When reconnected to the host, the sound was 5.1!
I was never able to reproduce this behavior, and I never saw the popup status message.

It seems there is going something wrong with this combination that might be related to the handshake.

Steps to reproduce

With the FireTV Moonlight and Sunshine host it always gives 2.0 sound (except once, see bug description).

Sunshine runs latest version v0.23.1

It has the same behavior when using normal audio speakers or Steam streaming speakers, so 2.0 sound in this combination, 5.1 in other combinations.

I can't find any sound related logs in sunshine.log

Affected games

Full desktop, all games.

Other Moonlight clients

PC

Moonlight adjusted settings

Yes

Moonlight adjusted settings (please complete the following information)

Default setting, but set Sound to support 5.1.

Moonlight default settings

Yes

Gamepad-related connection issue

No

Gamepad-related input issue

Yes

Gamepad-related streaming issue

Yes

Android version

Android 11 (Fire OS 8)

Device model

Fire TV Stick 4k max 2nd gen

Server PC OS version

Windows 10 19045

Server PC GeForce Experience version

3.28.0.417

Server PC Nvidia GPU driver version

560.94

Server PC antivirus and firewall software

Screenshots

No response

Relevant log output

No response

Additional context

No response

[drsno]

After a few hours more, I found a method that works for me:

• Plug the FireTV Stick in the receiver (in the TV does not work)
• In the receiver settings, find the correct HDMI input and set the HDCP from "Auto" to "1.4" (those are the only options).
• Start a new Moonlight stream and instead of Dolby Digital 2.0, I now get PCM 5.1 sound.
  I now also see the status message in the Moonlight UI when I start the stream every time.

So, the bug seems to be that Moonlight fails to start the full handshake process with Sunshine in the 'normal' setups. Maybe this 'workaround' helps to identify the issue. It does not seem a definitive solution, because UHD content requires HDCP 2.2.