This fixes bug #3531635 - "DREAMWEB: doors don't play "open" sound when
opening".
In addition, the resultant code is simpler and should better match the
original behaviour and a basic playtest has not shown any regressions.
This should fix bug #3528164 "DREAMWEB: missing sound
effects/music cues during main title" by preventing repeated calls
of SFX id 12 being lost if the next call is made before the sound
handler has cleared the previous one.
This change should have no functional change, but makes the sound code
more decoupled, modular and readable, prior to attempting a fix for
bug #3528164 - "DREAMWEB: missing sound effects/music cues during main
title".