How can i use the built-in microphone at the same time as using my earphones? (they have an external mic attached)

I am recording stuff on garageband and i need to hear other tracks as I'm playing, but I have to play them through external earphones so that the they don't overlap onto the one I'm creating.

The wire for my earphones has an external microphone attached to it, however, I have no desire to use this microphone as it is ridiculously sensitive and picks up even the slightest vibrations when the wires rub against my cheek or my t-shirt.

Therefore, I need to use the built-in microphone when recording. Except that, whenever I plug in the earphones to the iMac, garageband automatically switches to the external microphone. I went in to the Garageband preferences, and saw that it was set up for recording through the built-in mic, which is strange because input is clearly coming from the external.

So, I went into the iMac System preferences > sound > input. There are three options here: Internal microphone, Line-in, and Soundbooth 3.0.

What I found is that whenever I plug my earphones into the iMac, the option for 'Internal microphone' changes into an option for 'external microphone'. Now, I am fairly sure that an 'external microphone' is EXACTLY the same thing as a 'Line-in', but I'm not so sure now, so please feel free to correct me if I am wrong.

How do I stop the iMac from changing 'Internal microphone' to 'External microphone'? It is very important that I am able use the earphones and the Built-in Mic at the same time.