I have an Asus K50IJ (Bestbuy) laptop and have issues with my sound. Speakers themselves work fine but when I plug into the headphone jack it auto mutes the front channel and no sounds comes out of either the speakers or the headphones. If I then unmute the channel I get sound from both the speakers and the headphones.
I'm very sorry to tell you, but my laptop has the exact same issue on Windows. I tend to believe that the audio hardware doesn't allow us to play over two device connections simultaneously, and the only way to remedy this would be by a hardware mod. But that seems near impossible so you might be better off trying to connect a hardware mixer or a USB headset/headphone which would create a separate audio device which does allow you to play audio on both simultaneously...
My guess is that you could try to run Windows on your laptop to see if the same problem occurs there.
Realtek HDA here... From a DJing perspective this is a bad thing to discover. :-(