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Come on, you spent $5 for an app. For a working app! This is a pretty low price. Actually, it's basically nothing. Of course you can't get support and are not entitled to feature requests at such a low price tag!
$5 is not cheap by app store standards. It's a fair price for an app in that class. As to whether you're entitled to support or feature requests, you can make an argument both ways. One could generate some word of mouth by following up on (simple) feature requests. In one of the apps, Fourier, the reviewers say that the horizontal frequency scale is off by a factor of 2 (but the frequency display elsewhere is accurat…
Second, I understand your frustration. I love building things, and sometimes I wish I could do that full time, but grad school is a 70 hr/wk commitment for me.
If the app is not useful for you without the missing feature, Apple does allow returns. I'm not quite sure how it works, but I see a few returns a year on my reports.
Also, I've open-sourced the hardest part of the app, the audio managing aspect, as Novocaine (GitHub.com/alexbw/novocaine), and along with the great package NVDSP, anybody could replicate the basic functionality of my apps with a few weeks of learning and effort. It'd be great for the audio app ecosystem, too!
I'm also interested in hiring a part-time developer to help flesh out the top-requested features, if anybody has ObjC coding experience. That'd make many more updates possible.