sound keep playing through speakers when plugin headphones

i installed ubuntu 10.10 on my new Dell Vostro V1014, when i plug in a headphones sound keep play through the speakers only.
i tiried to "upgrade alsa" as i saw in some link when i searched the web but it killed the sound totaly, my brother fixed it to me but i still stick with the main problem which is the headphones dosen't work, and sound keeps playing with the speakers.

thanks.