Keep internal speakers on when earphone jack plugged in

I just bought a new speaker set for my macbook pro. It speaker set plugs in via earphone jack. I noticed that while the when the speakers are plugged into my computer via earphone jack, my macbook pro internal speakers turn off. Is there a way for both the internal speakers and my speaker set to both be on?

I don't know about both being on for the same thing (not 100% sure I understand why you'd want them both on instead of just one or the other) but you may want to look into a program called SoundSource which makes it really easy to swap what audio is fed where. You could even leave system audio on your onboard speakers and "output" audio to your line out. check it out here:

Download SoundSource for Mac - Switch your audio input sources easily. MacUpdate Mac Audio Software Downloads

Hope that helps some.