Remane sound_scene_handle to playback handle.
sound_scene_handle was a part of scene so we could see code like scene-
often in the same function.
If I understand things correctly, in audaspace lingo, the
playback_handle corresponds to a Reader while the scene_sound
corresponds to a Factory.
More cleanups will be done here later, but changing this now because my
brain hurts trying to remember which is which...
ce0d3112e4