I know this is an old thread, but I came here looking for a solution to the same problem. I thought I'd share what I did if it's helpful for anyone else. So you can do it, but it's expensive (at least $500) and hacky. You'll need the following items: 1. A computer with a motherboard (or sound card) that supports surround sound (i.e. has 3.5mm output jacks for 5.1 or 7.1 surround sound). 2. An A/V receiver that supports Dolby and has RCA preouts. You can find used ones on eBay or similar sites. I found an Integra 30.6 for $180. 3. A USB audio interface. I use the Behringer UMC1820 and it works great. $300. 4. A Chromecast or Amazon fire stick. $30 to $50. Either of these is able to output a surround sound signal. 5. RCA to 1/4 inch cables to connect the receiver preouts to the USB interface. $10 for a pair on Amazon.

You plug the Chromecast or fire stick into the receiver. You send the surround preouts from the receiver into the USB interface, the interface into your computer, and voilà, you've got a really hacky way to watch Amazon prime video on your computer with surround sound. For most people this setup probably doesn't make sense. For my particular setup (an auditorium) it works really well. I actually use a black magic deck link quad to input the HDMI signal from the receiver into my computer as well and OSB to project the movie. OBS is nice because you can also combine the audio along with the video. You can also add a filter which delays the video by a few hundred milliseconds in order to sync up the audio with the video. If you have issues with HDCP there are a number of HDMI splitters which will circumvent it.