It appears both next-gen consoles are launching with some features missing. While PS4 promises to add support for MP3 eventually, Microsoft has been caught having to promise another key feature for audiophiles in a future firmware update.
Apparently, Xbox One will not support Dolby Digital at launch. The reasoning? Microsoft simply didn't have time for it. "This was a SW scheduling issue pure and simple, and I know people are disappointed, but we will have it," Microsoft's Albert Penello said on NeoGAF (via IGN). To be clear: "Dolby Digital is coming post launch."
There are other issues regarding surround sound support, specifically with HDMI pass-through. Xbox is offering a support page to help work out some of kinks you may discover when you set up your Xbox One.