Just picked up the same mobo....I had the same problem until I installed the RealTek HD Audio Manager software on the Asus CD. Now if I plug my headphones in, the speakers turn off, and vice versa. When you have the software installed, it will ask you when you plug your headphones in, what you just plugged in, and give you the correct option. It should be straightforward.

Hope this was helpful

EDIT: Here are the settings you're looking for:

Click on Speakers, then in the upper right corner, click on Device Advanced Settings. Make sure mute the rear output device is checked, as well as Tie up same type of input jacks.