Since most of us aren't using TVs in the classroom anymore, the question seems to be coming up often about how to connect speakers to play videos. Usually the teacher wants to be able to play videos from a DVD/VCR as well as from the computer, and perhaps from an iPod.
You could, of course, have a separate set of speakers for each of these purposes, but it's easier and cheaper to have one set. To do this, you can connect the DVD/VCR audio output to the speakers and the computer audio output to the audio input of the DVD/VCR.
To connect the computer audio output to the DVD/VCR, use a 3.5mm Stereo Plug to 2 RCA Plug cable connected to the speaker (or headphone) output of the computer.
To connect the DVD/VCR to computer speakers, you need another 3.5mm Stereo Plug to 2 RCA Plug cable connected to the audio output and a headphone "gender changer" for connecting the male end of that cable to the male end of the speaker cable. Instead of a "gender changer", however, you can use a headphone splitter which also allows you to plug in your iPod without unplugging anything.
Of course if you use speakers with RCA inputs, such as the Behringer MS16s, you can just connect the DVD/VCR audio output with the RCA cable that came with the DVD/VCR.
If you want to show videos from your iPod and your DVD/VCR has a second "line in" connection (other than the one you are using for your computer), you can use an iPod video cable that connects to the headphone jack or to the dock connector. Of course if you have a newer iPod these cables won't work, you'll need to buy an expensive one from Apple. Or plug your iPod into your computer with a USB cable and show the videos using iTunes.
As to video to the projector, computer video goes over a VGA cable and the DVD/VCR video goes over an S-Video cable, composite cable, or component cable.
That ended up being a longer post that intended, and a bit of a commercial for monoprice.com (one of my favourite online retailers), but hopefully there was enough information there to get you set up.