Try as ravrob said, what I done for my folks was run an optical cable from their box to the soundbar and all is good.
If you are running it through the TV try setting Kodi to passthrough and if you can do it with the TV too all the better.
Sorry guys, I do not need to know about the physical connection btw the Nvidia and the Soundbar but the settings within Kodi 17.6 in the audio and anymore sections that will allow me to get Dolby Atmos audio out of the soundbar.
It depends on your tv and sound bar. Try various ones, there aren't that many. I have my Samsung connected to my home theatre via optical cable but cannot take advantage of the passthrough with my Shield. I gave up on that. But again, try and see if you can get it.
I used to have a HTPC with Optical sound output. If I connected it directly to my sound system, I had 5.1 but that was the only way for me.