If you’ve already got an audio cable hooked up from the DVD player to the TV set you’ve got a couple of choices. You can disconnect the audio cable from the DVD player to the TV and hook up your computer speakers (no sound will come out of the TV’s speaker), or use a splitter so the mono audio still goes to the TV while the stereo audio goes to the speakers. An inexpensive RCA Male to two RCA Females splitter will do the job. (An adapter changes one type of connector to another while a splitter takes a signal and splits it into two identical signals.)
My preference is to use the splitter because it gives extra flexibility. If I’m just watching an ordinary DVD then I’ll just use the TV’s built-in speakers and leave the speakers unpowered. But if it’s an action movie with lots of audio effects or a musical I’ll turn on the external speakers.
The first photo shows a gray colored splitter cable plugged into the DVD player’s white (left/mono) audio jack. The white cable from the adapter is plugged into the splitter and the red cable on the adapter is plugged into the red audio output jack (hidden underneath the gray splitter cable in this photo). The speakers are connected to the other end of the adapter. The other half of the splitter sends the left audio channel to the television, along with the existing yellow (video) signal.
Hooking up external speakers to your DVD player has an added bonus; the DVD player can also be used as an audio CD player. Just leave the TV set powered off. Put an audio CD into your DVD player and you can listen to CDs through the stereo speakers.