My built-in speakers won't mute?

I have a Sumsung laptop with Windows XP and Logitech speakers & subwoofer.

Yesterday, for some unknown reason, my built-in laptop speakers stopped muting when my Logitech speakers were plugged in. This is a big problem since my laptop speakers rattle a lot. There's nothing wrong with the Logitech speakers, they have a seperate power supply and they plug in where earphones normally do. I know the problem is with the computer because even when I plug normal microphones in, the laptop speakers still don't mute.

I don't want both sets of speakers to be on, only the ones plugged in. I've tried everything I can think of... can anyone else help?