I ordered a sound card of your brand on Amazon, it is functional and works very well, I use it with a 5.1 kit from Logitech (Z506) however I notice that there is only on Netflix that the sound comes out in 5.1, on all the rest of my PC (Amazon Prime Video (windows application), Deezer (windows application), Youtube and any other content only comes out in its stereo.
On my 5 speakers and the box only the main speakers and the box work, the front speaker as well as the two surround speakers are at a standstill. I set up the speakers with the windows program in 5.1, they are all recognized and work as it should and in the right place, the program installed with the sound card also works and is set up correctly too.
The two major platforms that YouTube video players use are Adobe Flash, and Adobe Air.
1. Flash 11 only supports 2 channel audio.
2. Adobe Air 3 supports multichannel audio.
3. Also HTML5 supports multi channel audio.
Most multichannel streams will be PCM no bit stream, as you need licensing from Dolby or DTS to encode/decode. Furthermore multi channel audio is only supported for TV devices, on all other devices has Multi Channel Audio( ) always returns false.
This is probably due to HDCP / DRM requirements by publishers.
Finally many smart TVs, especially Samsungs, will only send 2 channel audio from the optical output (again HDCP/DRM reasons).
So you need to be sure what player / equipment you are using. A smart TV using ARC (audio return channel) to a receiver will have multichannel YouTube audio.