How can I check my line-in does work (MBP late 2010)?
Launch System Preferences… from the Apple menu.
Click on Speech.
The left tab is marked Speech Recognition.
Set the Microphone from the default "Internal microphone" to "Line In".
Click on the Calibrate button. Then speak the phrases on the left e.g. "What Time Is It?"


If your Mac detects your line-in, you'll see the blue-green-red indicator respond to the volume of your voice from the line-in.

Have fun, and don't forget to mark the question as answered when you're done. Cheers.