ECS 755-A2

I have a new ECS 755-A2 (1.0) motherboard with an Athlon 3200+ processor.
The motherboard has the Realtek AC97 655 codec. The sound I get from the
rear audio jacks is very good, but when I plug in my headphones into the
front panel connector it sounds like the karaoke voice suppression feature
has been turned on and the sound is very poor and no longer in stereo. I can
get the same effect in the rear audio jacks by turning on this voice
suppression feature in the Realtek software, but it seems to be on all the
time for the front jacks. The motherboard has a 10 pin header and this is
cabled to the front audio jacks via the cable that came with my Antec case.
Does anyone have any suggestions on how to fix the front jack audio problem?

Thanks for any help,

Jack