Check and see if your card has a second ouput jack to plug speakers / headphones into.
Mine does, and I found out that the dang card is actually doing 4 speaker surround sound, and I can't turn it off. Flew for the longest time with only headphones connected, and always thought it was really weird that I could only hear flaps or guns sounds if Looked backwards....
I now fly with the reagular speakers / subwoofer as the Front L/R channel, and the headphones as the Rear L/R channel. However there are times (like 03:00) when I'd really like to fly sans subwoofer / front speakers, I just haven't figured it out yet...