Yes, you can’t record background audio (such as whaterver is being currently played by Spotify or Apple Music or phone calls). But your app can open an audio file from the user’s music library (Apple Music) and play it back as a stream or convert it to a wav file, if that is what you need.
The procedure is quite different to simply opening a file normally, though. I have just finished updating my fork of the excellent dRowAudio module to the current JUCE version to do exactly this. Check out the AudioPicker class and the iOS Audio Example. You can find my fork here: GitHub - atiliomf/drowaudio: dRowAudio JUCE module