Volume control on panel no longer has effect

[Dave M G]

Ubuntu Users,

It used to be that if I clicked on the small speaker icon on my Gnome 
panel, it would reveal a vertical slider that would control my 
computer's volume.

However, recently this has stopped working. I get the slider, but moving 
it changes nothing.

If I double click on the speaker icon, I get a window for "Volume 
Control" with a bunch of dual-sliders for various audio devices.

If I move the "Master" sliders up and down, nothing happens.

If I move the "PCM" sliders, that controls the audio.

I'm assuming, then, that the slider that appears under the speaker icon 
is tied to the "Master" channel.

Should it be tied to "PCM"?

Is "Master" deprecated?

How do I get the quick access to overall volume control that I used to have?

Thanks for any help or advice.

-- 
Dave M G