If you (or your team) opted in to test this beta feature, you can leave a comment on a specific time in a video file or audio file on dropbox.com. This feature does not work in the mobile app.
To leave a comment on a specific time in a video file:
Here’s a list of supported video file types that allow for specific time commenting.
To leave a comment on a specific time in an audio file:
A number is assigned to each comment ordered by its date or location.
The first comment is assigned "1," the second comment is assigned "2," and so on. A number is always tied to its comment and doesn't change if the comment is resolved or deleted. For instance, if comments "1" and "2" are deleted, comment "3" is still number "3."