Many people who create videos use the automated captioning option that YouTube provides. The resulting captions are certainly appreciated by viewers, not just for the most obvious reasons (people with hearing difficulties, or for whom English is not their first language). They are also great for watching video when it is inappropriate to have the sound turned up, or if you've forgotten your headphones!
this help from Google, detailing circumstances in which YouTube automatic captions might not be created, might be useful :
Essentially, if your video doesn't generate automatic captions, it could be due to one or more of the following reasons:
• The captions aren't available yet due to processing complex audio in the video.
• The language in the video is not yet supported by automatic captions.
• The video is too long.
• The video has poor sound quality or contains speech that YouTube doesn't recognize.
• There is a long period of silence at the beginning of the video.
• There are multiple speakers whose speech overlaps.