How do you switch between built-in audio and headphone audio without unplugging? Once I plug headphones into my iMac I no longer have the option in the sound preferences panel to output audio to the built-in speakers. I have to unplug the headphones to get audio output through the built-in speakers. Since the plug is on the back of the display this is inconvenient. Does anyone know of an audio app that allows me to easily switch between built-in speakers and headphones for audio output?
I think the condition has always been there.  I believe plugging into the headphone jack physically disconnects the sound output from the speakers.  I believe you will have to use bmike's suggestion to get the desired outcome.