MORTEVIELLE: Fix crash in sound mixer when closing the engine

This was due to the _speakerHandle being reused between its
initialisation in the SoundManager constructor and the destructor
causing it to have a wrong value when trying (and failing) to stop
the PCSpeaker channel in the mixer before deleting the stream.
MORTEVIELLE: Improve looping of the intro song

Using a LoopingAudioStream gets rid of the pause between each loop
(and simplifies the code).