How to mute internal speakers, not headphones?

[Daniel T. Chen]

On Fri, 2007-03-09 at 17:56 -0800, Charlie Zender wrote:
> 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.

Please attach the troubleshooting output from the top portion of
http://wiki.ubuntu.com/DebuggingSoundProblems

Thanks,
Daniel Chen