Ashley Yes I do see your point. Usually music based apps ignore the ringer-mute switch, Garageband for example. In fact pretty much every musical app, also apps like Youtube, and anything based on playing videos or music, podcasts etc. It's actually quite a standard behaviour. I suppose the reasoning is that you wouldn't be playing an on-screen piano if you didn't want to make a noise...
I get an email probably 2-3 times a week from users asking why the sound doesn't work on my app, and I have to reply with the default "turn the mute switch off" email. As this is the normal behaviour for an app, I think users get confused when it acts differently.