Kaizena is a web tool that allows you to add voice comments to Google Documents that are in your Google Drive, whether they are yours, or if they are shared with you.  To use Kaizena, you need to authorize permission for it to access your Google account.  Once that is done, you can open any document in your Drive, highlight a selection of text, and record your comments for students to listen to.  The video above does a great job of walking you through this process.  This is a great tool for providing audio feedback on students' written work.