secured site

 It's because it really is an AUDIO IN PORT, not a low voltage MICROPHONE PORT!

It requires 1 V pp line level voltages, and you bought an off the shelf dynamic microphone that only puts out a few millivolts.

You'll need something that brings the microphone levels up to line level. You could use an old cassette deck with MIC IN and HEADPHONE OUT in a pinch, or use a dedicated MIC amp that bumps up the signal level.

Old Macs like the LC III and such did use a basic MIC INPUT port, but later Macs switched to LINE INPUT so they'd work with signals coming from stereo systems, etc. Apple made a LINE LEVEL MIC with an extra long tip that gets power off an extra contact point in Mac models even as recent as a G4 tower .

The real question is, did Apple keep backward support for this microphone? If so, is it possible to find one? However, you'd lose your advantages of a headset microphone...

The best solution so far is the iMic which provides all required interfaces to plug in a standard headset. Details about the Linux support can be found here

Remember, the port is actually a STEREO INPUT port, so this may also mean you would need to use some STEREO to MONO adapters or find a software setting that will put one channel across both left and right as a MONO signal.