I have a computer that has a 3.5 mm audio jack on the front of it. The computer itself has no speakers, so this is the only way to hear sound. If I plug headphones into it, the audio properly plays through the headphones, and if I plug in external speakers it used to play through them as well.

Just today I turned on my computer and the audio no longer plays through the speakers, but if I plug in the headphones instead it works. The speakers aren't broken, as both the speakers and headphones work in my iPod and play music.

I thought that 3.5 mm jacks could not send data back to the computer, and the computer had no way of differentiating between different devices plugged into the jack. If this is true, how is it that the computer plays audio through the headphones but not through speakers plugged into the same 3.5 mm jack, and both devices are functional? Or is my knowledge on 3.5 mm jacks incorrect?

I don't believe drivers are important, as the same driver runs the 3.5 mm jack for all devices, but if necessary I can provide additional information.

Any ideas would be appreciated. Thanks!