Hi @ElizabethP
Welcome to the Sonos Community!
There are no products you can buy to improve things - although you can add surround sound speakers and/or a Sub to increase the overall immersion, movie dialogue, specifically, wouldn’t be heard from any of these, so they would not help with this issue.
It is probably mostly down to the movie’s sound engineering - director Christopher Nolan (Inception, Tenet, etc.) is well known for having difficult-to-hear dialogue in his movies, for example (sometimes on purpose). The fact is that no movies (excepting perhaps made-for-TV movies) are mixed to be experienced at home - in a cinema, the dialogue volume would be loud, and other sounds even louder. Some movies may get remixed for home release, but that’s really up to the distributer and how much effort they want to put into making a BluRay or streaming release.
In short, the best thing you can do is increase the volume.
I hope this helps, though I understand this is hardly the answer you were looking for.