Cool. You came at a rather bad time though, there hasn't been a lot of activity even in terms of new custom levels, and in any case, most of the means we currently have for playing custom levels do not support MIDI music anyway. But someone here may someday find some use for it, so feel free to post. (Perhaps Lix can make use of it? Not sure what it has and what it supports right now for music.)
My one suggestion would be that if it's music for puzzle games like Lemmings, I will go for something more bright and cheerful, or at least something with more motion and rhythm. Your song to me feels like the exact opposite of what I'd typically want for a Lemmings level music, but hey that's just my own opinion and taste.
