It appears that setting SPAudioEngine setMasterVolume to 0.0 isn't applied to new SPSoundChannels created after the fact?
Eg, the code flow goes something like this:
Create SPSoundChannel from file (SPSound soundWithContentsOfFile createChannel)
Play (volume as expected - default 1.0f)
SPAudioEngine setMasterVolume:0.0f
Sound Mutes (great)
Create new SPSoundChannel from another file (SPSound soundWithContentsOfFile createChannel)
Play <<< Can hear sound again? Should be 0.0f still, right?