MIDI music is now paused by pausing the parser(s) instead of relying on the
mixer streams to stop providing callbacks. This gives more consistent behavior
as external MIDI devices or softsynths now also pause playback.
Muting sound now sets the volume to 0 instead of pausing playback.
The BaseSound class does now only save the sound filename instead of a file
handle. When a new sound is started a new file handle is created, which
assures that each sound uses a different file handle and thus allows for
directly streaming sounds from disk.
This fixes bug #3475610
"AGOS: Wrong sound effects during intro of Simon 2 (DOS)".
