TL;DR: Yes
All Roku devices support passthrough audio, including 5.1. However, in order to get 5.1 audio for most services, you connected device must support DD+ (EAC3), not just DD (AC3).
If you have older equipment that only supports DD (and not DD+), the new Ultra (4800) can convert the DD+ to DD, giving you 5.1 in all supported apps/scenarios.