There are many threads here about microphone problems.
Can somebody please update the specs page to state exactly which type of 3.5mm socket is on each model? There are many types of 3.5mm plug and they are not always compatible with every headset or microphone.
Nonetheless, looking at the range of issues in the forum, people have mentioned Apple products having a different impedance and other people have mentioned problems with the size/shape of the 3.5mm plugs on some headsets.