Use internal speakers with others plugged into headphones jack I have a set of surround speakers, connected to my iMac through the headphone jack. I use a switch to control what goes to the speaker set (iMac, iPod, or MacBook/aux). So that I don't have to be constantly unplugging and replugging, I leave the audio cable in the iMac's audio port. This means that when I'm listening to something from another source, I can't hear anything from the iMac unless I unplug the cable.

Is there a way to force sound to come out of the internal speakers, even if there's something in the audio out port?
Pretty sure it can't be done with the built-in headphone jack. What you could try is getting a USB soundcard (one example, but there are lots out there for $20 or so) to plug your headphones into. That should give you two options in the sound preferences.

PS, if you option-click the speaker icon in the menu bar, you get a quicker way to swap inputs/outputs than going to the sound preference pane every time.