I am a visual learner who prefers reading over watching and listening.
I would like to turn some great video/audio learning resources into transcripts (free and public) and sell to people who love to consume texts as I do.
Is it legal to do this by giving credits to the original content creators?
Your rationale here is decent, but you run the risk of trampling all over copyright issues. And ending up in a legal soup as a result.
Giving credit does not absolve you of profiting off someone else's creation. And just because it's available for free doesn't mean there's no copyright protection.
Given all of that, consider a pivot for your business idea that involves working with the creators of these videos and podcasts to generate transcripts.
If you wish to discuss, send me a PM through Clarity for 15 free minutes.