Sorry, I should clarify. The core idea behind my app has never been done before; I made it because I desperately wanted it to exist. However, there are a number of independently-designed, $5+, iPad-only, fun-idea-type music apps that have had a lot more traction than mine. (One recent example is Fugue Machine[1].) I'm trying to figure out if there's some obvious flaw that accounts for the fact that apps like this tend to have several orders of magnitude more sales than mine.
Looking at my stats, it appears my main problem is that nobody knows my app exists. The # of views for all my media is just tiny, despite being on the front page of several subreddits, Product Hunt, and HN. Unfortunately, it seems this stat is directly tied to prominent writers talking about my app, and I can't seem to get that to happen. (My biggest sales boost by far was when a prominent music blog wrote a review for my app.) So I figured maybe something was wrong with the marketing pitch I usually send to publications. (Up to and including the way the app looks in screenshots.)