Netflix uses Dolby Digital + for audio. Your Stick (and every other Roku player except for the 2016 Ultra and some TVs) cannot convert the DD+ to standard DD, and your TV also doesn't support DD+. I believe by setting the Stick to Dolby, the Netflix app will only supply standard DD.
Nvidia Shield, Roku Stick (2016), and Ultra (2016), Insignia Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network. Samsung JU7100 4K TV, Yamaha RX-V583, CenturyLink DSL