多分これかな
http://community.linuxmint.com/tutorial/view/1236
http://askubuntu.com/questions/456425/alc892-low-sound-issue-fixed-with-changing-alsa-config-why-does-it-work

sudo nano /etc/modprobe.d/alsa-base.conf
ファイルの末尾に追加↓
snd-hda-intel model=generic