You're going to need some form of htpc, in whatever guise that means. I use that in the loosest sense of term as the lines between things are becoming quite blurred.
For example, you can sideload Kodi on the Amazon fire TV -
http://www.amazon.co.uk/Amazon-CL1130-Fire-TV/dp/B00KQEJBSW/ref=sr_tr_sr_2?ie=UTF8&qid=1427355786&sr=8-2&keywords=fire+tv which is amazons alternative to the apple tv. However, I couldn't comment on if that device did the HD audio (I'm sure you could ask in the kodi forums).
Or there are various android black boxes, but I have no idea how good any of them are.
Then you can start looking at the pc based solutions such as the intel stick
http://www.intel.com/content/www/us/en/compute-stick/intel-compute-stick.html which is basically a full computer that is the size of USB stick that just plugs in to the HDMI port on the tv.
Or a bit bigger, there's now a whole range of NUC pc's such as the gigabyte brix
http://www.gigabyte.com/products/list.aspx?s=47&ck=104 which are bloody small!
I guess it just depends on what you want and if you are looking at something that may do 4k or not. Kodi/xbmc standalone install is very good now, so you don't have to faff about with anything else computer wise if you didn't want to.
Other options are something like the popcorn hour or the wd boxes, but I've not used them so i can't comment on if they will do HD audio.