I'm genuinely curious why you say to get a lawyer for all music apps as I can't really see why this one would necessarily need a lawyer. Are you talking in terms of music licensing issues or patent violations, or other?
Licensing rights, because if you develop an app that makes money out of playing music it is gonna make some people uncomfortable, even if it the music of the owner of the app. There's gonna be C&D letters even if the app proves to be licit.
This makes no sense, there are no licensing rights involved because no music is being provided. It just plays files provided by the user. Are people sending C&D letters to Sandisk for selling MP3 players, or the hundreds of music player apps for Android?
There are already a huge number of apps out there doing the exact same thing, as far as I am aware zero have been sued/threatened out of existence over this.
A few jobs ago I was working on a video player for an embedded device and the UX team mimicked the CoverFlow interface gimmick.
We had a C&D from Apple within a few months of showing it in public.
I would assume there are other patents. For example, sampling the cover art to decide on the background color. Was that an Apple thing? I don't know.