Go to Amazon, and look up the exact movie that you are streaming. The audio stream varies by title,and you have to look each title up one at a time in order to see what gives for audio streaming. A download might be 5.1 while streaming may be 2 channel for the same title.
So, here is Iron Man 2 (HD)
So, a ways down the page, you see 3 possible formats:
TiVo 5.1 HD-WideScreen
Portable 2.0 (SD?)-WideScreen
Connected 5.1 (+ = with audio exceptions) (Video undefined)
I'm guessing a Panasonic BD-60 is a "Connected Device" but not "5.1 enabled". So, what you get from a "Connected Device" is completely (audio & video) un-defined on this page.
It seems that when you are inside the Amazon app on the device ... if you select this HD video, it does the initial negotiations. It then plays whichever version it wants (maybe not the HD version). Looking at the website, it's obvious which ones have been played, and which ones have never been started. I've been testing with the Iron Man 2 - HD version, but the website says it's never been started ... only the SD version has.