Hi Kim!

Unfortunately, I think the music course has been removed, at least maybe temporarily as Khan prepares to port huge amounts of their data.  Hopefully one day we'll see something pop back up.

Thankfully, Khan Academy's videos are still on their YouTube, and you can check them out here: https://www.youtube.com/watch?v=wQHcz7U01M4&list=PLSQl0a2vh4HDFvmGd8eL5PJusJqrNZ0ge

I also found this post from a user who suggested a video they found that might be a good addition to music theory.  Here's the post if you're interested: https://support.khanacademy.org/hc/en-us/community/posts/206432537-Music-Theory

Hope this helps!

Elijah