Don't know about YouTube, but if you go to the Tolkien Professor channel on Apple podcasts and scroll all the way back to the very start (2009) you can find them. As far as I can see the oldest thing Corey posted (that's still online) was is "How to Read Tolkien and Why" lecture, followed by his Hobbit series.
I assume this series led to him writing the Exploring the Hobbit book, which eventually led to the Exploring the Lord of the Rings series we've been in for 7 years now and will probably be in for the next 20.
BTW, Corey, if you're reading this, I'm curious. A year or so ago I went back and re-listened to these when I was reading the Hobbit with my daughter. I've been listening and participating in various Mythgard academies for years now and so feel pretty used to your style - those original Hobbit lectures seem highly prepared, as opposed to your usual (now) relaxed and relatively free-wheeling style. How prepared were they? Did you script them? Or at least have a super detailed outline? Or was this just the result of having taught through the books multiple times in the classroom previously?
Bruce