It is currently impossible for me to lower the sound with the media keys on the keyboard (Audirvana Mac Version 3.5.44 (3574)). It only increases. This is specific to the software because with macOS alone it works very well. Does anyone else have the same problem?
Try resetting the NAVRAM on your Mac. Shut down, then press Option + Command + P + R keys as you restart. Keep the keys pressed until you see the second Apple logo and then release.