You can follow these steps to add a comment an existing VoiceThread slide: 

1. Enter the Canvas site containing the VoiceThread. 
2. Find the VoiceThread. Most likely, it will be under Modules or Assignments. It might be helpful to full-screen the VoiceThread, which you can do by clicking on the Open in a new tab? link (circled in red below).

"Open in a new tab?" appears near the center-top of the embedded Voicethread display in Canvas.
3. Click on the VoiceThread (circled in red below).
example of how to open an existing VoiceThread

4. Click on the postcard-display button that is in the lower-right corner of VoiceThread (circled in red below). 

5. Click on the slide to which you want to add a comment. 

example of selecting a slide to edit

6. Click the Comment button (circled in red below).

example of how to click the "Comment" button in VoiceThread

7. Select your commenting option and follow the prompts to record your comment. Please consult the instructions for your assignment to see if you can only use certain comment-types (e.g., audio). 

the comment-type menu has several options: text, phone, audio, video, and file-upload

8. Your comment will appear on the left side of the VoiceThread in a square with your initials.