I found musicxml a nightmare, no matter what application you use there's always a problem with something missing, importing but not exporting, one app reads some parts correctly another app don't read it at all, one app will import it then export as something different.
Midi is a Standard that works from app to app but xml there's no Standard adhered to, it's all different from app to app.
Every company seems to have their own version/language, like communicating in broken English, pidgin musicxml.

http://www.musicxml.com/for-developers/

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