When exporting NWC files to midi, it's a good idea to delete any muted staves before doing the export.
The reason is that muted staves will still export their entire contents, albeit with velocity for each note set to zero.
Deleting the muted staves before export will have three significant advantages:
1) The resulting midi file will be considerably smaller.
2) There will be less chance of running out of polyphony.
3) With large numbers of staves, dense MPC's, etc. there will be less chance of overloading the resulting midi stream.
Fred Nachbaur