Testing revealed that Recordare needed another beta before releasing the final version of MusicXML 3.0. A new Beta 6 version is now available at:
Beta 6 has several bug fixes in the musicxml.xsd related to some of the new 3.0 features, including the play, system-dividers, hole, and key-accidental elements, along with the new unplayed and niente attributes. We have also renamed the sound IDs for the harmonium, nylon string guitar, ganza, and shekere instruments in the sounds.xml file.
Please let us know if there are things we should be adding or changing to better meet your needs. We already have one documentation fix in for the final release thanks to beta 6 feedback. The MusicXML mailing list (www.recordare.com/musicxml/mailing-list) is the main place for for these discussions, but please comment here or email feedback privately if you prefer.