You may also need to do this. Using a Terminal="Applications->Accessories->Terminal"
Open this file for editing:
Insert this line at the bottom:
gksudo gedit /etc/modprobe.d/alsa-base.conf
Save. Close. Reboot. Check your levels in alsamixer.
options snd-hda-intel model=acer
Press <F6> to select the correct soundcard.
Press <F3> to show playback levels. <F4> selects capture levels [or use <Tab>]
Use the left/right arrow keys to select and up/down arrow keys to change levels. <M> to mute/unmute.
Go to "System ->Preferences ->Sound" and make sure the correct soundcard is default.