The MusicXML format includes a standard set of instrument sounds to identify musical timbres. The sounds.xml file contains the standard sounds that are included in MusicXML 4.0. The required id attribute lists the MusicXML sound identifier string used by the instrument-sound element.
Applications may wish to provide a modified version of this file where the element content includes an application’s method for identifying sounds.
You can view the XML file in the Community Group Report: