After it was brought to my attention that distributing sheet music without first consulting the copyright holder(s) is technically infringement, I contacted Christopher Larkin to learn what his policy is. After some back and forth, he told me the following:
We encourage fan remixes/arrangements provided they're free. So upon further thought, it's fine if you want to release your arrangement of Hornet for free on Musescore :)
This is a good thing for this community; as long as you're never making any money off an arrangement/remix/performance, he is okay with it.
However, the moment you make money off something, even if it's something as trivial as ad revenue from YouTube, then Christopher Larkin has every right to take it down. In fact, he still has every right to take even free stuff down, but I don't think that he will exercise that right.
So if you plan to make money off a work derived from Larkin's, you'd better consult with him first. If he doesn't get back to you, chances are that he's still figuring things out; a lack of response is NOT implicit approval. If your project will not generate money, then you're almost certainly good to go, and you shouldn't need to receive explicit permission.