Thanks for getting in touch.
MusicKit is not something new. The ability to play apple music content already available in iOS 10, not everything is so simple.
VOX for iOS uses BASS audio library to play audio. This allows us to play almost every audio format out there and also allows applying custom signal processing our users appreciate VOX for - Parametric EQ, BS2B, level histogram (waveform), frequency analyzer and in the future - Replay Gain and Tempo/Pitch. This also allows us to enqueue tracks from different sources in the same playback queue - our core philosophy.
Apple Music API, on the other hand, is run by using proprietary MPMusicPlayerController that is fed by music track id’s from user’s Apple Music Library that can only play iOS supported audio formats, and the actual audio file is hidden from developer. Thus, there is no way VOX audio engine can obtain access to audio file it needs in order to play.
It is my hope this information helps! Have a great day!