Can only generate quarter notes using the MusicXML interface

[jogama]

Reproduced in the linked gist: https://gist.github.com/jogama/8b5e08b6533375f2f92c912d0fe71719

Thank you for all the hard work you've put into implementing the MusicXML standard.

I am not certain if this is a bug, or just my using the interface incorrectly. If it's the latter, I can make a pull request to update the guitar/piano example to close the issue, once I am corrected.