Netflix was having issues this past week, with movies that were supposed to be Dolby Digital Plus (DD 5.1) only streaming in Multi-Channel. Things have changed since Friday for Amazon that I have found, and some stuff with Netflix also. Make sure that your Roku is setup properly for Surround Sound in its settings is a start. Also as for the HDMI cable, you do not need a $100 HDMI cable, a $5.00 cable from Amazon or Newegg will work fine.
As for Amazon, same thing as Netflix this past week, they were having issues sending Dolby Digital 5.1, and looks like it has also been fixed as of Thursday. I addressed this on Tuesday with Sony, Amazon, Netflix, and of course Sony's answer was "Here is a list of our new line of equipment that you may be interested in." In other words, it is not our fault. Amazon got right on it, and credited back the money for Transformers Revenge of the Fallen, and were right on top after explaining that I only got Center channel and the Denon AVR-1612 that I have, would state "Multi in" regardless what we played, but only got the Center channel.
I did a reset on our Sony BDP-S590, just to make sure nothing we wonky, but after waiting, it really came down to the CDN that Netflix uses from Amazon, that was at fault.