Hi,
Given your OS and setup, my first suggestion would be to check the MIDI sound level. In XP, you can go to "Sounds" and then the "Volume" settings, and there's a separate level for MIDI.
When Notation opens up your MIDI device, it sets the default volume at 50%. There's a long story behind that, but suffice it to say it's at 50%, and you may want it higher
Some other MIDI players have their own internal MIDI synth, and so volume levels may vary when you play the same file on different players.
ttfn,
Sherry