How to hook subwoofers to a computer?

My new computer speakers won't work when I hook them up to the computer!?

• (My descriptions of the plugs will seem simple and basic because I am not the most techy person in the world.) I purchased a set of speakers for my computer, ones with a round ,light green, cylindrical plug to hook into the computer. I hooked them up to my computer in the matching socket(light green) but no sound comes out when i play something with sound, for example a movie trailer or a music video. There was no installation cd accompanying the speakers and according to the box they should work as soon as you plug them into your computer. I know that the speakers themselves work because they play music when i hook my IPod up to them. They just don't play sound when I hook them up to my computer.

• Answer:

  Try going to Sound under your Control Panel and right click then select 'Set as Default' on the device.