iMac microphone not working in applications

Hi all,

For the last few months my iMac's microphone has stopped working in applications (rather annoying when I am away from home and want to Skype friends and family!) I have configured the microphone numerous times and the iMac itself picks up my voice and commands. However, when I am on Skype or other external applications, the other side cannot hear anything I say. I have tried to do the Skype test call, my microphone isn't picked up there either, so it is definitely my microphone not their sound.

Please can anyone help me?

Thanks in advance!