The MusicXML Document Type Definition (DTD) is divided into several files as listed here. The Partwise and Timewise files are the top-level DTDs for musical scores. The Opus file represents collections of works. The Container file is used with compressed MusicXML 3.1 files, and the container.dtd file contains information about the compressed file format. The sounds file maps between MusicXML 3.1 sound IDs and application or library sound IDs. There is also a separate DTD for representing Standard MIDI Files in XML.
You can download all the MusicXML DTD and XSDs as a single zip file from the W3C GitHub repository:
You can also view the individual DTD files on GitHub:
For a description of recent changes, please see the MusicXML Version History. For a listing of all the features in the MusicXML 3.1 format, please see the MusicXML Alphabetical Index.
MusicXML 3.1 is also available in a W3C XML Schema Definition (XSD) as well as this Document Type Definition.