Instead of creating the mixer externally and then passing it in, it can just be made within the class.

I was statically setting the sample rate to 44.1Khz when we default to run at 48Khz.
This was causing audio to sound much too low.

After calling enqueue the callback would fire before we had assigned g_mixer a value.
This would cause a fun crash to happen.

and s/\bNULL\b/nullptr/g for *.cpp/h/mm files not compiled on my machine