...and that's what truly matters, in the olden days prior to OS X, the "Pass Thru" function of the "Line in Input" was easily setup, nowadays unfortunately, this is not as easily done, yes you may still access your Audio MIDI Setup located in .../Applications/Utilities/, but even that doesn't always provide the necessary amplification results needed to effectively use a Mac's Line In input (e.g. with a generic non-USB microphone, or in order to listen to your tunes or videos saved in your iPods thru your Mac's speakers.) I highly recommend LineIN and its developers at Rogue Amoeba Software, LLC, a company with which I am not associated in any way whatsoever, except as a happy customer, the price (FREE) cannot be beat.

I've successfully used this app in my Mac laptops:

15" MacBook Pro C2D 2.16 GHz

15" MacBook Pro C2D 2.33 GHz