Made addAudioObjToMix() a static function

This required including copying over the this pointer to the mixerThreadArgs struct, as static functions only have access to static class members. With the this pointer passed explicitly, non-static members can be accessed. Though no errors are thrown, there is no playback. Likely due to a race-condition on the output buffers, which need to be locked with MUX.
removed sleep in iOS callback, fixed playback issues