why are headphones only device for sound output

long story...

bought iMac in Feb '11, used successfully until June '11. downloaded update for Lion, lost all audio. Apple couldn't explain it, so they replaced the logic board. worked fine for 3 months, then, one day I was using garage band and computer had a nasty freeze. rebooted, and the only output device for computer was headphones and the built in mic was picking up no sound. Apple couldn't explain it. took in for second repair, replaced logic board and this time audio cable. brought it home, computer worked for 15 minutes, until I used garage band. once again, nasty freeze, restarted, same exact issue. Apple went in, did a full interal reading of my computer, diagnosed that everything was fine software wise, but insisted that it was a hardware issue or had broken the audio cable by repeatedly inserting headphones into headphone jack. dispatched a service tech to my home to replace audio cable (again) and to no avail. tech suggested to Apple to replace computer, and they did. now I have a brand new iMac, has worked beautifully for 7 days, and once again, I sit down to use Garage Band, and wouldn't you know, nasty freeze, reboot, and yup, all audio is zapped. computer states that there are headphones plugged in, but makes no sound, and once again, picks up nothing from the built in mic.

only thing I can guess is that maybe I am shorting out the cables by plugging in headphones to the headphone jack and using a microphone thru the microphone port on the back of the computer!!? mind you, this is the same exact set-up I have used for at least 6-8 yrs on different Macs that I've owned, and even on the one I purchased in Feb of 2011, and nothing has ever happened before.

has this happened to anyone??

does anyone have any ideas what is going on..?