In general, the Roku does not perform any audio processing. Unless you have it down-mixing to stereo, it just passed whatever audio codec is being sent. Depending on the streaming site, that is most common either standard Dolby Digital, or DD+. If you are streaming from a home media server, you could also have DTS audio. And a few sources might have PCM or FLAC. Again, the Roku just passes the audio though. I have my Ultra and 4 connected to Yamaha AVRs via HDMI, and have never noticed any audio difference on my local media compared to playing the physical disc.
Since you would be connecting the Roku directly to the TV (for 4K HDR support, since your Marantz sounds like it doesn't support HDCP 2.2), any audio processing that might happen would likely be done by the TV, not the Roku. I have no idea if your TV will pass DD+ (or any other audio other than DD) back to the AVR without modification. Some TVs will not pass anything except DD out of an optical connection from an external source. Using ARC may or may not offer different output options. Since you don't state what is bothering you with the current audio, I can't offer much else in the way of suggestions.
First, I use a high speed WIRED network. The Roku 3 is plugged into the AVR along with the TV and Blu Ray player. The Marantz SR6005 supports ARC and HDCP as well as all surround formats except Atmos. When looking at the specs for the Roku 3 it states the audio support is digital while the Premiere Plus states 'Dolby Digital'. I use the ARC port on the LG OLED for connection to the Marantz which shows the surround format being used on the digital display. It seems clear the TV is passing the surround formats and since upgrading to the LG the audio improved slightly. The audio quality varies and when watching films on HBO GO the volume has to be cranked higher than with Netflix; why I'm not sure though I suspect it has to do with the HBO servers which aren't nearly as efficient as Netflix. Where the audio itself is concerned, some content is fine while others sound muddy via my Definitive monitors and surround speakers which are top notch. Obviously upgrading to the 4K version Roku should improve video though I'm wondering if audio would improve too as I said.