There are some free options available to generate closed captions from Audio/Video.
Amara − Amara is one such tool. It is very easy to learn and provides the best platform to create captions from scratch.
Youtube − Youtube is the most popular tool which is available for free. It has great features to create captions.
Subtitle Edit − This is a free tool which has to be downloaded on your laptop and desktop and you can start using it even offline.