Last week, Recordare released a new Beta 4 version of the MusicXML 3.0 format. You can download everything from the zip file at:
Beta 4 includes some bug fixes, as well as an updated sounds.xml file with 887 standard instrument sounds.
We are getting close to the final version of MusicXML 3.0, so please let us know if there are things we should be adding or changing to better meet your needs. The MusicXML mailing list (www.recordare.com/musicxml/mailing-list) is the main place for discussing MusicXML 3.0. But please comment here or email feedback privately if you prefer.