The master volume keeps changing while I'm listening on youtube music in the browser. The OS is Debian Jessie. I adjusted the volume in the GUI and using the terminal (with "amixer set Master 40%") and it later changed repeatedly regardless the way I've changed it previously. There are no other applications running besides the browser.
If pulse audio is the default and applications are changing it, you may need to fix that. Check these two config files for "flat-volumes"
If it exists and is not flat-volumes=no, change it to be no. This may solve the issue. I would suggest adding more info...does the volume change when you reboot?, etc