este é o bug, isso acontece depois de um encerramento grave.
https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/1264213
fixed it. now sound works properly. but wasted a lot of time doing nonsense for days.
added this line to the file "/etc/modprobe.d/alsa-base.conf"
"options snd-hda-intel model=ideapad" and then
"sudo alsa force-reload" and then
reboot
for more info visit this thread => http://ubuntuforums.org/showthread.php?t=1316634
edit:
the above solution is not a permanent solution.
Installing NVidia driver fixed the problem. This looks ridiculous but works.