Hey looking back through my setup I realized I have something a little different. I have my htpc with hdmi going to my tv and then toslink (optical) audio feeding my surround sound.
It looks like to get my setup work had to install a custom audio driver. It’s called “aaf Optimus” which allows me to select Dolby digital as an option under “default format” in the audio properties menu.
Not sure this applies to you as my setup is kind of convoluted. Anyways when I watch YouTube the sound is just plain 2 channel audio and when I play something from Netflix it does pass through surround. Games are also in surround sound.
Just wanted to double check everything and give you a heads up. Good luck.