Record with MacBook Pro headphone jack
All of Apple's computers feature a headphone jack or sound output of some kind, but only a few of them offer a recording input. The iMac includes one, as well as the 15-inch MacBook Pro and the Mac Pro desktop, but you won't find one on the MacBook Air, Retina MacBook Pro, or the 13-inch MacBook Pro.
So what's an audio geek to do (aside from invest in a decent USB sound card)?
Well, in the case of the 13-inch MacBook Pro (old and new), Apple includes the option of a switchable audio port than can be used as either an input or an output. Most people (myself included) see the headphone logo on the side and assume the audio spits out in just one direction. Not so.
To make the switch happen, go into your system preferences and select the control panel for Sound. At the bottom of the window, you'll see a drop-down menu for "Use audio port for." Switch it up, adjust the input level slider above it, and you're all set.
Another quick way to set this is to hold down the Option key while clicking on the volume slider on the desktop's menu bar. You'll get instant access to your audio port settings.