i tried this with my acer aspire 4935G - didn't help at all (i thoought maybe it works for other aspire too).
what worked for me, however:
ticking all volume controls for HDA Intel (Alsa Mixer) and checking if one fader turns up the headphone volume.
and it worked - "surround" controls the headphone plug volume, "front" the volume for the integrated speakers.
couldn't find a way to automatically turn off speakers when headphones are pluged in, tho.