Problem with Mic

I recently bought some new Ipod earphones that contain an inbuilt mic on them. I used to use the iMac's inbuilt microphone for skype calls however when I plus my earphones in it automatically switches to the iPod mic. The problem is I dont like the iPod mic, and my friends have complained that it muffles my voice or creates distortion, is there anyway of switching it back to the inbuilt iMac mic while having the earphones plugged in.

Are the earphones connected to the iMac via the audio out socket, or usb?

With the earphones plugged in, try this. Open SysPrefs > Sound and click the Input tab. Select Internal Microphone. Talk something to see if the indicators light up. If they do, then you should be all go to use Skype as you and your friends want.

there plugged in by the audio socket, when I go to sys prefs > sound > input Im only given two options external microphone and line in, when I unplug them "external microphone" switched to internal microphone.

Okay, so line in cancels internal microphone. Darn! I think therefore your best solutions are:
1. a USB headset,
2. disable the mic connection of the iPod earphones (snip one tiny little wire - not sure which one - don't own an iPod).