Adding voice comments to your students’ documents can be an efficient way to give them feedback on their work. Voice commenting allows you to give full, customized explanations to your students. It also helps to have students literally hear the comments in your voice as opposed to trying to interpret the tone of your written comment.
http://www.freetech4teachers.com/2018/03/how-to-add-voice-comments-to-pdfs-and.html