Headphones turn off internal mic

Hello all,

My wife has a new mac book air. She using Skype and tries to plug in some nice and comfortable sony headphones so that she can listen to an office conversation and type down the minutes. When she plugs in the Sony headphones this knocks out the internal mic, even when she unplugs the headphones, and I believe even with a restart of Skype.

When she plugs in the iphone headset, it all works again. I understand the concept of the 3 way plug that the iphone set has.

Does it always work that the normal stereo headset turns off the internal mic? That doesnt seem to make too much sense.

Thanks,
Justin.