Updated setPlaybackRate method to change defaultPlaybackRate

[tinotk]

As stated in Issue #221

[scottschiller]

Thanks for this, sorry for very late follow-up - I like the idea, but want to work in a defaultOptions or similar option for this. I have an existing pattern in a few places for volume and others IIRC, and I can make this work for you without having playback rate on one sound require that all others change.