How to mute internal speakers, not headphones?

[Charlie Zender]

Hi,

My laptop now play sound through its internal speakers even when
the headphones are plugged in.
Previously, if something was plugged into the headphone jack, then
the laptop would only play through the headphones. Now it's playing
through both! This is disconcerting to the library's other patrons!

I'm running fully patched Edgy on a Dell Inspiron 9400.
My only sound application is Amarok.

How do I mute the internal speakers while keeping the sound playing
through the headphones? All of the controls I've found change both
at the same time.

Thanks!
Charlie
-- 
Charlie Zender, surname at uci.edu, Department of Earth System Science
3228 Croul Hall, UC Irvine, Irvine CA 92697-3100. (949) 824-2987 :)