i had the same problem with a much different sound card but the solution may be the same or at least worth a shot.
run alsamixer from a terminal as root and mute the headphone control (and associated headphone volumes if it allows) and unmute the others (PCM, Line, etc). I also had to mute a couple of 3D controller options. May take a bit of messing around with... it took a while for me to get mine sounding good.
Hope that helps,
I am root. If you see me laughing you better have a backup.