Your post has ‘slightly’ confused me - are you saying you’ve tried a different A/V receiver and it’s working, or when you mention the A/V receiver are you referring to the Beam and is it only a PC/Sound-Card you have connected it to?
 

If so, I suspect it’s either sound-card related, perhaps it’s driver/software that needs a setting changed… or possibly a jumper setting (unlikely) on the sound card itself. The Beam will only play what is sent to it through the optical/HDMI-ARC connection, so my thoughts are that it’s most likely something at the sound-card/software side of things. I would maybe check it’s manual/manufacturer’s support desk, or their forum, to see if you can glean any info from there.

Are you sending a DD or DTS 5.1 signal? Sonos only supports DD. 

There was another thread recently where the pc “test” mode sent sound to each channel individually but it wasn’t actually capable of sending DD sound. 

I tried my TV as a source and my PC. On the PC I’ve configured to send Dolby Digital 5.1 in the device properties of the soundcard’s optical output. I’m using the Asus X-99a Motherboard which, to my knowledge, supports surround-sound. At least in the Windows Output settings I can choose between Stereo, DTS, and DD 5.1. I’ve set it to DD 5.1 and I can hear sounds when testing.

But what concerns me, in the realtek hd audio manager I can only find options for DTS-connections, not for DD. Maybe the onboard soundcard isn’t DD 5.1 compatible after all?