why. can't I hear. a sound out of the IMac?

That jack often goes sideways, sometimes reinserting the plug a few times helps, a few have carefully probed around with a wooden toothpick to fix it... you'd think they could spend more than a dime on a nice jack.

Has anything been plugged into the headphones port lately?

In Sys Prefs>Sound>OutPut, what is chosen, is Mute checked? What volume setting?

Indeed. :(

Any luck yet?