This was by far one of the favorite apps of the whole school year. You can buy all of the letters for pretty cheap, but you can also win tickets to earn new songs. There are different genres for each of the letters, and my students loved them all.
If you're curious what the videos look like, here's my students' favorite one, the letter B. Beware, these are the types of songs that kids can listen to a million times a day, and they get stuck in your head and they're hard to get out. I still love them though. I think you can see all of them on youtube, if you need a preview.