Since no app was revealed I can't write much here. Playlists are often just text files so a non-playlist file would be ignored and we'd just make a new playlist for use by the mystery app.
All the audio filters that I’ve found require input sounds to be on a music application playlist. Is there someway for me to either find a filter without this requirement, or get a file onto a playlist? I’m forced to only use my iPad running 11.2.1