[Since EBCDIC]

I'm setting up a new work computer today, and the CEO strolled by and tossed me a pair of Koss SB/49 headphones with built-in boom microphone. So I plug one into audio out, the other into, well, you know.

Audio out works as expected for headphones. Audio in, however, doesn't seem to "see" me plugging or unplugging; System Preferences always shows a line "Line In - Audio line-In port". The input level is always nothing. I've played with the input level slider; no joy.

The audio in plug looks as though it's a stereo plug (even though there's only a single boom mic). Does the MBP expect a mono plug? Is this the problem?

Help, as always, gratefully accepted.

[Since EBCDIC]

Okay, okay, it seems that the headphone's audio out plug needs to go into a USB dongle.

That isn't intuitively obvious. Grrrrrr.

[jogi]

The mic in plug is not mic in but audio in, i.e. it needs amplified current not provided by a microphone.

[mduell]

As you correctly noted in your first post, it's a "line in" port not a "mic in" port. It needs a line level signal, not the signal from a microphone.