I'm keen to hear more about the guitar learning helper you guys built! I've been planning to build a similar thing for a little while now. Care to do a blog post on what you guys did?
Regardless, you can easily transpile a codebase to support those browsers.
If you aren't doing that already, you probably should be. Otherwise you're supporting a codebase that is quickly going to become very dated and (in comparison to a more modern codebase) much messier.