Chosen Solution
When headphones are inserted into iMac headphone jack, not only do I still hear the built-in speakers but NO audio can be heard via the headphones. Have tried several different headphones all with the same results.
Go to system preferences and click on Sound. Click on the output tab and choose line out. This should send audio only out of the headphone jack and not out of the speakers. If you still get no sound on the headphone jack, you may have another problem.
Very likely your jack itself is broken. Ebay has 3.5 mm usb to audio jack (hear and speak) but the one I bought was such low quality, the sound was unendurable. The volume was also very high on the lowest setting. It may be a mac-friendly usb to 3.5 mm audio jack will work and have better sound. Ultimately you fix the jack itself.
