Each time I log in to Ubuntu 14.04, using fast user switching, my volume gets reset to about 10% and muted. I have to unmute and slide the volume control back to where I want it. This even happens if I just select "switch user," but then log back in as myself, without actually switching users.
I put this in my ~/.profile file:
amixer -D pulse set Master 50% unmute
It sets the volume correctly upon logging out and back in, but will not prevent it from muting when switching users.
As a temporary solution, i suggest you put that line into your
.bashrc. The only problem is that it will run every time you open up a terminal.