Comment 2 for bug 301127

Sorry, I tagged the wrong bug when I was marking duplicates last night. There are around 15 bugs that report the same issue: speakers not being muted when headphones are plugged in. By comments on a couple bugs, this seems to be an across-the-board issue stemming from a line in /etc/modprobe.d/alsa-base:

A comment in bug 332542 indicates that this will fix this issue:
"it's in adding to:
/etc/modprobe.d/alsa-base
following line:
options snd-hda-intel model=auto"