From time to time, say once every other day, without touching anything on my computer, the volume lives a life of its own. When this happens the OSX volume icons are displayed onscreen and the volume goes either to full blast or to a complete mute.
The volume keys on the keyboard are then unusable, but the OSX volume slider (top right) still works. Trying to restore the volume with the slider however is counteracted by the spontaneous volume changes, until after a minute the uncommanded volume changes stop.
When quiting Boom, volume control is regained instantly.
I've notice this behaviour in iTunes and while watching YouTube in Firefox.
2012 iMac 27, OSX 10.11, Boom 2 1.3
The next time this happens, could you please check your Sound Preferences and tell us what is selected under 'Output'? Also. do attach console logs of Boom from around the time it happens so that we can see what's going on.