audio in

i have a 2007 13" macbook and i am having issues with the audio in port. i have bought 3 different headsets that plug in to the headphone jack and the audio in jack and all 3 the microphone would not work but the headphone work just fine. i found late last ight that when i would plug them into just the audio that i could use the microphone if i talked into the left earphone. i'm trying to figure out why that is happening?