-A portableaudio Device. (iPod, iPhone, MP3 player, etc.)
-A standard headphone jack size audio cable. Male to male. ("mini", or 3.5mm)
-A computer with built in speakers.
Step 1: Understanding the "Jacks"
The headphone jack is another name for the audio output. So the headphone jack on your iPod is outputting the audio to your headphones that receive the audio to play it through the tiny speakers.
The microphone jack is another name for the audio input. This is where you plug any type of device that you need to output some kind of audio into, like microphones. When you plug a microphone into your microphone jack, or input, you are sending the audio from the mic and to the computer.
You can not plug headphones into a microphone port (input), and receive audio from the device. And you cannot plug a microphone into a headphone jack (output) and have the device receive audio from the microphone.
Now that you understand all of this, let's move on.
Step 2: Making It Work
Now if you play something on your iPod, you're probably not going to hear anything coming through your computer. So the way that you fix this on a Mac is with a program called LineIn.
LineIn allows you to play audio from other devices (like microphones) live through your speakers.
You can download it from here: http://www.macupdate.com/app/mac/11333/linein
[I'm not as familiar with Windows computers, but I believe there is an option somewhere in your Microphone Properties to turn this on, but I'm not 100% sure.
There's a forum about it on Windows here: http://www.vistax64.com/sound-audio/222117-talk-through-mic-hear-out-speakers.html
(I'm not affiliated with them at all)]
When you finish downloading and installing LineIn, open up the program. On the "Input from:" section, choose "Built-in Input: Line In". Set the "Output to:" to "Default System Output". "Built-in Output: Internal Speakers" should work as well.
After you have those set, click "Pass Thru", play some audio from your device, and listen to it playing through your speakers!
Have fun with this. If it does not work for some reason, or if you have any questions, please leave a comment below.