Want to use macbook as amplifier

When I use to have a windows laptop this worked. Im a DJ. I have a cable that comes out from the mixer and into the mic jack of the computer. On my PC, I would be able to hear the turntables out of the computer speakers, then connect external speakers. On mac, this does not work. I can see that it is picking up the sound, but it will not play the sound live. This is a big disappointment, so I need some help. thanks a lot

This sounds very similar to a problem someone else had recently. I recommend reading that thread and his final post... he had to resort to calling Apple and they did some troubleshooting that resulted in adjusting some settings in GarageBand to get him going.
http://www.mac-forums.com/forums/showthread.php?t=89776

In garageband you can use the monitor function on the right hand effects panel.