I have had similar issues with Roku Ultra, but I also have the Streaming stick+ and Streaming stick 4K.
I think the root of the audio problem happened when they upgrade the Roku OS to 11.0. I seem to recall no audio issues with my Roku Ultra (2020 version) until it was updated to 11.0, particularly when watching an older movie. I suspect older movies use an audio codec that is somehow changed or no longer there. For example the 1996 From Dusk to Dawn, George Clooney movie. It is currently on Amazon Prime and the Roku channel. On my Roku I really have to crank up the volume just to barely hear the dialog. Is there a channel separation issue? I use headphones on stereo. Regardless of the audio setting it matters not, same issue. However I can watch the same movie on the same monitor, etc, except I use my Amazon Cube TV device on Prime, and the volume for that movie is no problem as all - very clear audio and volume. I have had this issue several times with various movies.
https://www.imdb.com/title/tt0116367/?ref_=ttls_li_tt
Movie link
PS, Roku OS 11.0 did seem to fix the HDMI HDCP 2.2 issue with screen presenting an error code, and having to reboot. So just fix the audio issue please.