I have this problem. I have a "fix", if not a solution. It requires that you install "indicator-sound-switcher". This loads in your system tray at boot-up and allows you to pick from a drop-down list of your speakers, headphones etc. It does not solve the issue of why the sound settings keep reverting to default. But, it does make it very easy and quick to switch.
See below for an example of it in operation:
https://youtu.be/yeBmi-DfumY
To install it, open a terminal and type the following:
sudo apt-add-repository ppa:yktooo/ppa
sudo apt-get update
sudo apt-get install indicator-sound-switcher