I'm using Sonos Beam Gen 2 on my PC via HDMI, but the Dolby Atmos sound is too low.

Not sure there is much that can be done, from a Sonos perspective, than what you’re already doing. Sonos plays, essentially, what it is handed. If the Sonos signal received from your splitter isn’t loud enough, I’d suggest reaching out to the company that made it. Obviously, with your PC’s settings at max value, there isn’t anything at that end that can be done, and you’d like to not have to adjust the speaker’s volume every time, which leaves the middle piece of the puzzle. I’d also check with them to see if there is some sort of software/firmware update that may need to be applied to it, and how that process would work. 
 

Sorry there isn’t more I can suggest. At least from a Sonos perspective, you’re doing the right things. 

Hi ​@Blue Windy 

Thanks for your post!

I agree with ​@Airgetlam - if you get Atmos reported in the app, you are already doing everything right.

I do find this report a bit strange, however, as I can also get Atmos from my PC to my Arc Ultra, but it is one of the loudest sources I come across - I usually have to turn the volume down to about 10% for games!

I don’t see how this could be anything to do with your direct connection (no TV), but I suppose it is possible. I guess the way to find out would to try it with a TV present, to see if it is any different.

You are using the Dolby Atmos app from the Windows Store, I take it?

I am sorry that I cannot give you any particularly sound advice in this regard, but I can say that I think you should start your search for an answer on your PC - whether in the Sound settings or the Dolby Atmos app, I do not know, but given my own experience with Atmos from a PC, it seem like the best place to look.

One point is that I am using a 40-series nVidia card - if you are using a brand other than nVidia, I suppose that could explain the difference in volume. The Properties page for the audio driver may have some additional options, depending on the brand.

I hope this helps.