You can leave a comment on a specific time in a video or audio file on dropbox.com. To do so:
All video and audio file types that can preview in Dropbox allow for comments on specific times.
If the file belongs to someone with a Dropbox Professional, Business Advanced, Enterprise, or Education account, other people with those account types will be able to comment on a specific time.
If the file belongs to someone with a Dropbox Basic, Plus, or Business Standard account, comments can only be left at :00 and the time will be grayed out.