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.
Roku Stick (2016), 4 and Ultra, 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-V679, 60+Mbps CenturyLink DSL