Why is there a 5.1/7.1 Dolby Digital test on YouTube when for a standard setup is 2-Channel stereo?

YouTube has test videos like THIS one for testing your surround. My setup is as follows:

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• HTPC running Windows 7 with S/PDIF out for sound and HDMI for video directly to my TV

• Pioneer Eliete AV receiver to process my sound.

• Que es la mente humana. Kodi (XBMC) to handle my local media

When I play my media I have it all set to code it to Dolby Digital this includes upmixing 2 channel stereo. But unless I am running it via Kodi I get all Stereo. Even when running Netflix. It is my understanding that Netflix cannot run 5.1 audio it claims to have when using your browser because Silverlight does not support it. So I assume it will be the same when streaming any video through a browser like YouTube, the videos cannot play it in true Dolby Digital, only Stereo.

Is there something I am missing, is it possible to have play the YouTube video I linked in Dolby Digital?

How to Confirm 7.1 Sound to Headphones from PC?

Hi, I wanted to confirm my setup was correctly setup for 7.1 surround on my speaker headphones based on the following:

-Sennheiser HD 598

-Astro Mixamp 2014 Edition

-My PC with Mobo-Onboard Optical Port

I have my HD 598 headphones plugged into the mixamp. Mixamp is plugged into the PC via optical TOSlink cable (into mobo port for Optical). Note that it is also plugged in with a USB Cable for Power to the PC. I have selected Digital Audio (S/PDIF) in Win10 for the default sound (not the 'Speakers' option that is available from the USB connection, if I am understand this correctly)

Now couple of questions:

-How can I test for 7.1 surround? The mixamp is supposed to provide virtual 7.1 after receiving digital signal from computer and ultimately produce 7.1 for my HD 598s. I can't test via youtube because no videos properly output surround from youtube. Is there a go-to testing file/site?

-Is there anything wrong in my configuration? I thought I read something about needing a certain signal/encoding format sent from my computer to the mixamp in order for it to do it's 7.1 processing correctly to the headphones, an I'm unsure if my onboard audio from the MOBO is outputting the correct type of Dolby signal or w/e is required? (Does this mean I need a soundcard in addition to the mixamp?)

Thanks