Laptop Speaker "on" when plugged into external spe

[Aristotle11]

On my T42, when I plug in external speakers to it, the internal laptop speakers turn off. But when I do the same thing with my X31, the internal laptop speakers remain on. Is there any way to fix this?

I want to be able to control the sound just be turning the knob on the external speaker. Plus, when I turn it down I can still hear it on the less "high fidelity" laptop speakers. Even at low volume I'd prefer the high fidelity external speakers not be filled with such impurities from the laptop speaker.

Thanks,
Aris

[Aristotle11]

Apparently I'm using the line out. Since I use a dock, the line out is the only think to which I can attach the external speakers. Any way to disable the internal speaker when an external speaker is connected to the line out on the dock?