How can I check my line-in does work?
I'm interesting what software should I use and how.
It is MBP, late 2010. I do have an external working mic.
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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.
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You could try using LineIn.app from rogueamoeba. It allows to pass-through audio from line-in to speakers (normally disabled in Mac OS X). I did use this app quite a few times to play music from external source. Should do the trick for you.