^HDMI still can, so SURE. That HDMI->Optical adapter would be called a receiver, ROFL!
Astro may come out with a new Mixamp featuring HDMI input (or someone else), but by the time you get a new device you might as well get one supporting DTS HX with the support for overhead positional channels. I'm just as surprised as you, but I guess accessory backwards compatibility is the same story as previous-gen games.
In my mind, it's just one more solidifying thing against the Xbox 1. First, it was the anti-used game policy, I know a lot of publishers would've loved that and I know they reversed the decision, but it was the principle of it. Then, there was the online DRM checking, they also reversed that but it adds to the strikes on principle. This adds to the strike against M$ because they advertised better servers and no hacking for the years of Xbox Live I've paid for, but I've never seen any improvement in those areas, just saw a fee to unlock online multiplayer. I was starting to shrug off all that, thinking maybe I'm being too harsh and the XB1 really would deliver a new, premium experience, and the new controller and Halo were calling me... But BAM! We wanna shove new accessories and paid subscription entertainment like HBO and Netflix and Sports down your consumer gullet, We're Microsoft, the big money stash that never dies, you're welcome!
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If I reeeeally wanted, it so happens that I could still get (IMO) the equivalent of DH by hooking the HDMI up to my Yamaha receiver for Silent Cinema mode.
*Shrug*