setting up speakers

Please forgive a stupid question. I bought two little speakers. I plugged
them into the back of the hard drive in the green plug (the other audio plug
didn't do anything; the sound still came out of the hard drive). The sound
out of the outside speakers is barely audible. If I unplug them, the sound
through the hard drive speakers is loud. I've played around with all the
volume buttons; it's very loud on the hard drive speakers and barely audible
on the outside speakers. Am I supposed to do something to switch over from
one to the other? Am I supposed to turn of the speakers on the hard drive
somehow?