PC Audio - Undesired Split Signal

Hi all,

I own a Toshiba Satellite A355, running Windows Vista, with plenty of RAM and hard disk space.

Ever since I purchased it, I have had the following problem: The internal PC speakers continue to emit sound despite having a jack plugged in. This happens regardless of whether it is headphones or an external speaker system plugged in. The signal seems to be split between both internal and external - in other words, the external speakers emit sound simultaneously.

The first few times I booted up the system after purchase, Windows asked me whether I had headphones or a line out plugged in to the jack. Then, it stopped asking, and I have no idea how to get back to that screen.

What's likely going on here? Can I correct this by re-downloading drivers?