Nor can I. I find it indistinguishable with Music so far, and I'm wondering if it's a glitch that will resolve itself hopefully. If not, consider me underwhelmed.
Edit: Actually, tried a few different tracks, enabling/disabling. I can hear the difference, but it just sounds like a different EQ setting, no increase in sound stage like I'd hoped (which I do find quite remarkable on supported video).
It's not that I'm an old curmudgeon either. I absolutely can hear the difference on TV+, Disney+ etc, to the point where I was delighted to hear tvOS will support it eventually.