This could be a corrupt preference file. Try this, Whilst in Finder click on Go in the menubar, then click on Go To Folder.
Now enter this text, ~/Library/Preferences now press Go.
A new Finder window should open, scroll down the list to com.apple.systempreferences.plist, move it to the Trash.
Now restart your Mac, once it has booted up open System Preferences> Sound and see if you can now adjust the sound on your Mac. If you see a duplicate System Preferences.plist with a random selection of numbers and letters after plist, this should also be trashed.
If this doesn't work then download the 10.14.5 Combo from here, https://support.apple.com/kb/DL2000?viewlocale=en_US&locale=en_US if you already are on version 10.14.5 it will do no harm to install this, it will reset some caches and system files and may help resolve your problem.