iMac EMC2805 Internal Speaker option Missing in Sound Preference Menu

The headphone jack has a switch element within it it looks like that’s broken or messed up some how. Did you plug in a headphone which was an odd length or size? It could have damaged it.

Luckily Apple changed the design so you can replace the jack! It’s just not that easy. You’ll need to take the logic board out to get behind it to remove the jack assembly.

This is the correct behavior! The assumption is if you plug in the headphones (left plugged in) you don’t want the internal speakers to also be active. This is how most PC’s work, and how all Mac systems work. My TV’s and stereo also work this way.

Here’s a closer look on how the headphone jack works

The simple act of plugging in the plug breaks the short to the internal speaker lines.

If you want an either or both with the headphones plugged in you’ll need to get your self a pair of wired (using either the headphone jack or USB plug) or wireless Bluetooth speakers (with the headphone jack you’ll need a spitter). You’ll then need to manually to enable which you want on. With Bluetooth the menu bar can be used.

In your case I’m suspecting the switch contacts within the jack are messed up. The easiest way to fix this is to replace the jack iMac 21.5" Headphone Socket and Cable (14/15)