I am certainly no audio expert/afficionado and cannot tell any discernible difference between these different Settings. (too many concerts in my younger days).
Like you, my Insignia RokuTV also does not have a Sound Level or Volume Mode. (different manufactures, different Roku OS builds, different features available to adjust). All I have is a Sound Mode - Theatre, Speech, Normal, Big Bass, High Treble, and Music to choose from. Those other previously mentioned modes of Leveling, Night, etc, are however available on my Roku devices.
If your question is more regarding the DD+/Passthrough, the only Audio Setting I see on my RokuTV that mentions DD+ or Passthrough are located in my Settings. (Settings...Audio..Digital Audio Format).
Within that Setting, I see 4 options.
4. Custom (Dolby Digital and Dolby Digital +, as well as a DTS compatible AVR option)
Maybe these are available to you as well. I really don't remember seeing these in previous Roku OS builds but I just use the TV speakers and only mess with the Audio Settings if encountering a Channel/app issue during troubleshooting.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941