The ati HD audio device is for HDMI output on card.
Something tells me something is wrong with the headphones.
Did you try a different pair and are you sure they are fully plugged in.
Also try wiggling the wire near the hdphone plug and also slowly
pull it halfway out.
If doing this you get full sound from phones then it is a bad plug.
Very common for a bad plug on headphones.
also when you installed video card maybe you partially pulled out front panel