On an Asus X551MA I was having this problem as soon as I replaced Windows with Ubuntu 14.04. Even if I muted the speakers in aslamixer, they would come back on unexpectedly.
When I got to step three in this troubleshooting procedure, that seems to have fixed everything. It's a long complicated command which installed various packages, so I'm not sure which part(s) were really necessary:
sudo apt-get update;sudo apt-get dist-upgrade; sudo apt-get install pavucontrol linux-sound-base alsa-base alsa-utils lightdm ubuntu-desktop linux-image-`uname -r` libasound2; sudo apt-get -y --reinstall install linux- sound-base alsa-base alsa-utils lightdm ubuntu-desktop linux-image-`uname -r` libasound2; killall pulseaudio; rm -r ~/.pulse*; ubuntu-support-status; sudo usermod -aG `cat /etc/group | grep -e '^pulse:' -e '^audio:' -e '^pulse- access:' -e '^pulse-rt:' -e '^video:' | awk -F: '{print $1}' | tr '\n' ',' | sed 's:,$::g'` `whoami`