HOPKINS: Attempt to fix crash when skipping movie

Before starting an "Swav" sound, check that its sound handle isn't
already used. If it is, stop it. When skipping a movie, it still
(incorrectly?) goes through the motions of playing the remaining
frames, including starting any movie sounds on the way.

This would cause us to lose the handle to the sound that was
currently playing in the "Swav" slot, and I *think* that's what
ended up confusing things. I'm still not entirely sure exactly
what was happening.