How can I restore the sound on my Monitor?

I have a 14" Proview active matrix TFT LCD VESA Standard Wallmountable LCD Monitor from which I have had almost 5 years troublefree service.
I recently turned down the sound volume via the button located on the side of the Monitor and cannot now return it to it's previous state.

I have followed - to the letter - the instructions contained in both the user's manual and my computer to restore the sound but cannot make any headway whatsoever.

Can anyone help, otherwise I will have to learn to lipread!!