I have talked a few times lately about captioning video to make them accessible for all learners. I am not a Mac user, but, iMovie is built into each Mac computer and it can be used to caption videos. I saw it done at a conference recently, and it looked fairly simple to do. Apple has made a whitepaper with directions on how to do this. You can find one by clicking this link.
I think it is important to remember that all students need to be able to have accessible videos. Not all students can learn my merely hearing, some need to read that is spoken to them. Captioning is a simple step that will benefit your students.
iMovie does take a little longer to caption than using YouTube, but, it can be done prior to the upload and be built in directly to the video. One step that is different than editing captions in YouTube is the direct typing of word for word content. YouTube does find the content for you and you just edit. With iMovie, you will have to type as you listen. The steps are a little extra, but it is worth it in the end.