I rendered Byrd's "In Winter Cold" in three parts. When I used the xml output by Photoscore (music scanning program preferred by Sibelius), the converter seemed to be working, but output nothing, three times. I then exported the xml into NWC, using mxml2nwcc and edited it some. I saved it as nwctxt and parsed it through http://nwc2musicxml.appspot.com/ back to xml. That file worked fine in the converter.

Not too pertinent to this discussion, but Sibelius recommends Photoscore for scanning music while Finale touts Smartscore. I have full versions of both scanning programs and find that Finale accepts the xml files from Photoscore with far fewer errors than those from Smartscore. Go figure.