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The reason a single jack (with very few exceptions) can not be used as a microphone input and a headphone output is pretty simple. It would be putting out a signal (let's say an MP3) to headphones, but that signal would have to travel back into the microphone electronics, which would amplify it and send it out again. This would cause a feedback loop. The only way I know of for something like this to work is to either make it switchable, so you can use one or the other but not both at the same time, or to use a special connector for the mic, so that it doesn't connect to the headphone electronics, except for the ground.
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