You may use audio (with a microphone or telephone), video (with a webcam), or text to comment on a VoiceThread. To comment, you simply click on the comment button at the bottom of the VoiceThread page, and then decide which type of comment you want to leave. Click on the appropriate icon, and then follow the prompts to leave your comment.
For detailed instructions on each of these commenting methods, see the help article: https://voicethread.com/howto/how-to-comment/
We also have a video available to demonstrate commenting on a VoiceThread. You can see the video here: http://vimeo.com/30030761