added workaround for broken MIDI volume behaviour for Windows platform

playing MIDI music is broken since Windows Vista, as it sets the volume
for MIDI music also for all other sounds and music, which cannot be set
back to normal unless playing MIDI music again with that desired volume
(more details: https://www.artsoft.org/forum/viewtopic.php?f=7&t=2253)

=> workaround 1: always play MIDI music with maximum volume
=> workaround 2: never fade MIDI music to lower volume, but just stop it