OSX Yosemite change sound output while headphones plugged in
If you have a Macbook Pro with only one jack input (no separate inputs for microphone/headphones), try alt (option) clicking the Sound Menu in the menu bar and selecting "Use Audio port for" -> Input. This way the system will think that you want to use what's plugged in as microphone and not as headphones and you will be able to use the speakers once again.