plasma-pa Notifications Sounds volume is not adjustable on default 17.10 daily install without installing pavucontrol or pavucontrol-qt. Adding pavucontrol-qt to the base ISO would fix this issue and provide a better end-user experience.
This was supposedly fixed in 5.10 as per https://bugs.kde.org/show_bug.cgi?id=367688, but after testing 3 different computers, I beg to differ.