Handle fix for SDL Mixer

I've voided this out on other sound interfaces than SDL Mixer ones because I'm both not sure whether they need it, and not sure how to make them work with it if they do. I chose this option over the other 2 because the first I was fairly sure would possibly still have the issue, and the other I felt would still present issues from the possibility of clearing the channels in one thread while another is doing stuff with them... If the channel code in s_sound could be made thread safe somehow it could potentially be possible?

See #33 (closed) .