Comment 3 for bug 796617

found a solution, i used the settings advised here : http://www.backtrack-linux.org/forums/backtrack-5-beginners-section/41508-alsa-problem-after-successful-update.html

options snd-hda-intel index=0 model=acer-aspire-7730g

now, for the first time i have subwoofer, left and right speaker, all working at the same time :)
as positive side effet, volume never was so loud