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I have a PC with my RTX 3090 hooked to a Benq TK700Sti. I then have the eARC returned from the projector to the Beam, Sub, and two One SL. 
 

In the sound settings in windows, it senses that the max number of channels is 6, and also shows several supported formats, which includes Dolby digital 5.1. Additionally, I am able to go through setup and select correct correct positions and even enable the 5.1 setting. Everything seems like it should work. 
 

The problem is that when I actually try to test the speakers, the left and right channels are the only ones that play sound. If I click on the symbol of any other channel in the testing diagram, no sound plays. Also, if I play Netflix or something, the Sonos app says it is playing in stereo PCM. 
 

The 5.1 setup is identical for my Apple TV 4K and my PS5, and I get proper Dolby Digital 5.1 with those. 
 

Any idea what the problem could be?

Hi @RonPaul21 

Welcome to the Sonos Community!

I looked up the specifications for your projector, and the only audio format mentioned under it’s audio support section is Dolby Atmos:

Therefore, it’s possible the projector does not support MultiChannel LPCM audio, which is what the Windows audio subsystem would be providing it with.

If you play a local movie (not streamed) on the PC with Dolby Atmos audio and with audio passthrough enabled in the playing app or codec settings, do you then get surround sound? It seems like you should.

It would be surprising if the projector didn’t also support Dolby Digital 5.1 - the audio codec filter that Windows is configured to use can probably be told to re-encode any movie audio stream to Dolby Digital in real-time, prior to it going to the HDMI output. This may be worth trying too.

There are also some settings in the projector’s options menu that relate to eARC - it may help to try some changes there too.

Finally, there are sound cards that can encode all output (including that of games) into Dolby in real-time. Dolby Digital Live, I think it is called, and has been available on Soundblaster cards ever since Microsoft re-engineered the Windows Audio subsystem in Windows Vista (I think it was Vista, anyway). You would need a special cable to inject the soundcard’s output into the HDMI from the graphics card.

I now realise I wrote this in the mindset of someone with local files stored on the PC, rather than someone who might be using Netflix or similar. Are you using the Netflix application, or a browser window? Does it have any audio settings?

I hope this helps.

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