SL40 Volume Control not Working

I just noticed that the volume control on my SL40 isn?t working. Maybe it?s been this way for a while and I haven?t noticed. I?m hoping there is a setting that disables the volume control but so far haven?t found any mention in user or install manual. Any ideas?

From my dim memory, the install manual shows a setting of 0-255 for various volumes. Either 0 or 255 the volume will be controlled by the knob. In between, volume will be fixed at that setting.

Headphone level

Press and hold MON button for 3sec. Turn LARGE outer knob to display Headphone level. Adjust the small inner knob and select ?0?.
Setting the value to ?0? will slave the headphone audio level to the Volume Control Knob.
Good luck.

Thank you gentlemen....