Another NOOB Question - Surround Sound & Amplifiers


For my backyard movie theater I'm trying to get surround sound for movie watching. I have 2 amps, five speakers and one sub. Previously,  for stereo sound I'd run hdmi from a Roku 4 to a little Audio  decoder/extractor/splitter box to a Behtinger MMX882 (splitter/mixer) to the amps and then to the speakers. For some reason the box wouldn't separate the surround channels like  I thought it would. I was told elsewhere I would need an AVR receiver to split the channels. The AVR receiver i have is also an amp and the surround channel outputs are apparently powered, which means taking the surround channel outputs to the input on the Behringer or directly to the amplifiers wouldn't work - Or so I'm also told. How would I get the surround  to the Behringer or Directly to the amplifiers? Any help or direction to resources where i can learn what I need to know would be greatly appreciated!
mrlupin
I rum movies on my pole barn via a projector. I use 2-channel audio for it. Works well.  
Yeah, I hear you on that. To an extent I'm able to account for being outside because part of the viewing area is bordered by trees, a house and a large screen. Of course, that won't be enough to get sound as good as in a more enclosed environment. Thanks!
Mesch, I've done that for years and it's worked very well. I've always wanted to see what I could accomplish as far as surround sound outside though. It's been an challenge because I'm very much a novice. I think the missing piece of the puzzle for me is a preamplifier/processor with HDMI inputs, 1 HDMI output for video and then the pre-outs for 5.1 sound.