Surround 5.1 audio not working for games (only stereo output), setting doesn't survive reboot.

[DNKROZ]

Your system information

• Steam client version: 1726256783
• SteamOS version: 3.6.13
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: Yes
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

Hardware tested is a Steamdeck OLED 512 GB version using the official dock (sw updated) and a direct HDMI connection to a Marantz SR7011 receiver (no ARC being used), 5.1 audio has been tested and used multiple times, daily, with several other elements (Apple TV, nVidia Shield, PS4, PS3, Nintendo Switch... and so on...) and it's working fine so I rule out the receiver and cable completely. With SteamOS current stable version audio is 2.0 by default, I can get to Desktop Mode and change the audio properties of the current device being used (HDMI connection) and change those properties from Stereo to 5.1 mode, I can also test the individual output speakers and they work perfectly fine, when I get to Game Mode I see that the AV receiver is currently getting a LPCM 5.1 signal and all speakers are active (and also LFE channel), when I launch a game (I've tested with Alan Wake 2 (Epic), RDR2 (Steam), Cyberpunk 2077 (GOG) I get a 5.1 sound (RDR2 and Cyberpunk 2077 need to configure the audio in their settings but Alan Wake does this automatically), if I reboot the Steam Deck I'm still getting a LPCM 5.1 signal and games still function the same with the expected 5.1 audio.

Using SteamOS current Beta (and second RC, this is the thing that worries me most) I get the same 2.0 audio by default but redoing the same steps never end in a 5.1 working audio for games and it does not survive a reboot (2.0 signal after reboot). I can get to Desktop Mode and change the audio properties to 5.1 and the speaker test works perfectly fine but no 5.1 audio in any game (only stereo and even if the audio signal is still being identified as 5.1), there is no difference if the game is launched from Game Mode or Desktop Mode.

Steps for reproducing this issue:

1. From Game Mode get to Desktop Mode (since we don't have a way to change this from the Game Mode UI).
2. Select the correct audio device at audio settings and change several times from Stereo to 5.1 (for good measure) and leave 5.1 selected.
3. Go to extended properties (top middle icon if I remember correctly) and click the "Test" button, verify that every speaker works as expected (this works for me)
4. Launch any game from Desktop Mode or go to Game Mode (check the signal at the AV receiver and you should see a LPCM 5.1 audio signal) and launch any game and you 'll get Stereo output even if the output signal is still 5.1, doesn't matter if you configure the game with 5.1 audio (RDR2 or Cyberpunk 2077)
5. Downgrade to Stable version and retrace these steps and you should get a 5.1 signal AND 5.1 audio in game everytime.