Techhound
There is nothing stated or implied in the OS 11.5 release notes (user or developer) which indicates any such app-specific fix for the DirecTV Stream app's volume issues, for older or newer Roku models (Roku doesnt provide a changelog, just abstract summary of new features and/or APIs/API changes).
This is not to suggest that a change/fix in OS 11.5 doesnt/couldnt incidentally/also address the DTVS app's volume issue, just that it is unknown (and unlikely).
The most likely source for an app-specific issue is an update to the app/service itself, and for that you need to address it to the app vendor (DirecTV in this case).
Regardless, as OS 11.5 rolls out to different device models, and assuming the DTVS volume issue still exists, it should be apparent relatively quickly whether it provides any sort of incidental fix.
Finally, it should be noted that the volume issues with the DVTS app have been noticed on several platforms, which indicates a service/server-side technical issue, not a platform specific issue, so the most likely resolution is with app/service updates from DirecTV.