on my dual 1.8, i never found a fix (software) up to around mid 10.4 when i had to sell it.
that said, i went for a hardware fix by using logitech speakers with a remote volume control that had a headphone jack on it. that is, the volume control was a seperate device that controlled the speakers, but also had a headphone out jack that muted the speakers when headphones were plugged in. i set the g5 volume out to max, and used the logitech control for volume. many late nights of quake 4 spent this way...
g5 sound (analog) out > logitech volume control > headphones. worked great!
best of luck.