In XFCE on Leap 42.2 and up to Leap 15 if one has the onboard audio and a USB
DAC or multiple USB DAC's, the volume control on a keyboard or on a laptop
becomes what i would call disabled.  Masicly the volume control buttons dont
work at all and i dont know why.

Tested equipment:

Dell Poweredge T410
    C-Media USB audio in a USB dock
    Logitech H600 headset
    Logitech USB digital Speakers

Dell Optiplex 790
    Onboard Audio
    Corsair Void wireless

Dell Inspiron 3850
    Onboard audio
    Triton USB Headset
    Beringer USB Audio Capture

Thinkpad T500/T61/T60/T400
    Onboard Audio
    Logitech H600 headset
    Turtle Beach P11

HP Compaq 6910 
    Onboard Audio 
    M-audio Pre-USB

Home built PC 
    Onboard audio
    Logitech USB headset

Zotac IONITX F-E
   Onboard Audio
   NOX Audio USB DAC


I don't remember this happening on KDE but i didn't run KDE long enough to
notice it before KDE memory leaked.