What version of macOS are you running? Please click on the Apple menu in the corner of your screen, and choose About This Mac. You should see the macOS name, such as macOS Monterey or macOS Big Sur, followed by its version number. Thanks.
There are various ways MacOS can get into a state where diskImageMounter stops working. Have you tried rebooting? Did you recently do an OS update? Have you tried opening other .dmg files after that update or recently?
Also, check the file size, I just downloaded it and got 445502575
$ md5 Roon.dmg
MD5 (Roon.dmg) = 881c91bc76f1c1228dd8efd35373998f
I don’t think Roon publishes checksums so someone else needs to verify that as 1 unofficial checksum should not be trusted.