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We do this. It depends on the content of the transcript. If it's a person speaking about something it's going to flow naturally unlike a stuffed SEO paragraph. If it's related to the page it will help because of extra content and long tail keyword possibilities even if the main keyword isn't in the content. Don't be concerned about keyword density as long as you are addressing the keywords on the main page content. It's real content for the user and the engines, also handicap friendly for the hearing impared.