How to switch on and off the headphone jack on Mac OS (imac)

I have my iMac on a desk against a wall, and it's very inconvenient to access the headphone jack (and all the connections in back). The only way to do access the connections on back is by moving the whole computer. I have a hub for all the other connections, but it doesn't include the headphone jack. So I attached a short extension cable to the headphone jack, and I can plug in my headphones to that when I want to use them. When anything is plugged into the headphone jacks on my computer, the computer external speakers are automatically turned off. The problem is that sometimes I want to hear the computer speakers, and I can't find anyway to do so except by unplugging the headphone extension cord, which, as I mentioned, is very inconvenient. Is there a way I can switch on and off the headphone jack (and the external speakers) using the computer desktop interface? Thx.