output from your computer's audio output (usually green) jack. You plug a cable from there up to the input jack on your monitor.
I have these external speakers that I have been using for a couple of years that I started using from the beginning with my current computer since I first tuned it on; I have never used my computer without them. Just now, one of the speakers stopped working. So I figured that I would use the speakers on my computer and/or monitor until I could get out to buy a new set of speakers. But when I unplugged the speakers from the back of my computer, a red circle with a line through it appeared on the speaker symbol on my screen, like what happens when I press mute. When I click on it, it says that my speakers are not connected. I know that, I want to get the sound to come out of the speakers on my computer or monitor; how do I do that? I've tried right clicking on the symbol, that doesn't tell me anything. My computer is a Dell Optiplex 745 and I am running Windows 7.