This class represents the sound annotation. This is definitively detailed in:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 185, page 407.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 188, page 493.

This class is always an indirect object because it inherits from MarkupAnnotationElement which is always an indirect object.

System.Object
    WebSupergoo.ABCpdf14.Elements.Element
       WebSupergoo.ABCpdf14.Elements.AnnotationElement
          WebSupergoo.ABCpdf14.Elements.MarkupAnnotationElement
             WebSupergoo.ABCpdf14.Elements.SoundAnnotationElement

A sound annotation attaches an audio clip to a page location represented by an icon. Activating the annotation causes the viewer to play the sound. Sound annotations are deprecated in PDF 2.0.

The Sound entry references a sound stream object encoding the audio data along with sample rate, channel count, and encoding parameters.