How can I play the sound from the audio in through my speakers? I have got a Macbook Pro 15" that has both an audio in and out port. Now I would like to play the sound from an external device through the speakers of my Mac. 

How can I do that? I tried to just plug in the 3.5mm cable from my external device into the audio in port but I simply cannot make my Mac play the sound. However, in System Preferences -> Sound -> Input I can see that the Mac is receiving the signals. 

What can I do?
If you don't have Garageband installed you can also use Quicktime.


Open Quicktime
Go to File -&gt; New Audio Recording. You will see the following screen:

Click on the little triangle on the right and choose Built-in Input: Line in as shown in the screenshot.
Slide the volume under the record button up until you have the desired loudness.