Optical cables are not capable of volume control.  By that I mean, your PC can’t tell the beam to lower it’s volume.  I suspect with 2.1 setting, the soundcard is changing the volume that it sends to be Beam.  You can confirm this by watching the Sonos app and seeing if the volume is changing their when you are changing the volume outside of Sonos.    And of course, with the 5.1 Dolby setting, the sound card isn’t doing anything with the volume.

I don’t know anything about your PC and soundcard though, this is just a guess.

The Sonos app and the Windows 10 volume control are independent of each other whilst using SPDIF 2.1. I can set the Sonos App volume to 50% and then change the volume using Windows, this doesn't affect the Sonos App volume in any way. This is fine if all that I listen to is 2.1 stereo music/media.

If I set my soundcard to dolby 5.1, windows loses control of the volume and only the Sonos App can change the volume. This IMO is odd, hopefully some guru @ Sonos can explain why the app/Sonos behaves in this way.

Googling the issue highlights the as a common problem and points the issue to the Sonos App being a closed system and lack of dev on the Windows Sonos App. Not sure if this is true but it’s a real shame.

Anyone here with a similar issue as me?

Your information shows that it’s not a Sonos issue at all. Your PC is not controlling the volume of your Beam, as I stated.  All your PC is doing is changing the input volume it sends to the Beam. Optical cables are not cable of sending volume change commands.

There may be some Sonos users on here who can help with your PC and soundcard, but you might find better help on a media PC support forum

FYI, when using an optical cable with a TV, controlled by remote, the remote is controlling the volume on the Beam directly.  If the Beam is connected to a TV via HDMI-ARC/eARC, then the TV can send the command to change volume through the HDMI cable (CEC commands).

Hi @mikeyosm 

Welcome to the Sonos Community!

With Dolby 5.1, the PC is compressing the audio prior to sending it to the Beam. It’s also the case that the application doing this is not bothering to send the feed to the Windows sound mixer - this is why Windows has no control over the volume. To alter the volume, the Windows sound sub-system would have to decompress the stream, alter the volume, then recompress the stream to send it via the optical connection. This goes against the spirit of “Passthrough” where the audio is simply passed over to the audio device to handle.

In short, this is expected behaviour, and there is little-to-no chance it will ever change - though you’d need to ask Microsoft to be sure.

Contrary to this, when you play 2.1, you are in fact playing PCM stereo, which is uncompressed and how all digital audio systems/software handle audio internally, regardless of codecs used. In this case, the audio stream does go via Windows’ sound sub-system, therefore the volume can be adjusted in software.

I hope this explains it for you.