CentOS - Oct 2016 - Firefox and Chrome - dicking about with my volume.

I have Centos 7 installed on my laptop which works perfectly fine.

I use a variety of music apps including VLS, mplayer and squeezelite. They all 
work fine, and using the KDE mixer app I have the levels set just as I like 
them.

Why is it then that if i start up either Firefox or Chrome, whichever I start 
up first creates a new sound device which appears in the mixer, and then 
changes my master volume level.

Firstly, why the hell does a web browser have to create a new sound device, 
and why the hell does it alter the master volume?

Secondly, can anyone tell me how to take back control of my own music volume, 
and prevent both programs from messing with it?