Allow MH to have a master volume that controls the relative volume of each speak or play event.
Don't need to set volume if in mute or offline mode.
Merge remote-tracking branch 'remotes/hollie/master' into mh_sound_master
Merge remote-tracking branch 'remotes/hollie/master' into mh_sound_master
Merge pull request #257 from pmatis/mh_sound_master

Mh sound master volume