As soon as you answer an incoming call, the music / sounds will be paused on a Galaxy Note 8 (Android 9). On my Galaxy Note 5 (Android 5) the music is then very quiet, barely audible. Thus, everything seems to be okay. I use the Player component, the TaifunPlayer and the ExoPlayer in my apps (AI2, Kodular).
Note: As long you hear the ring tone from an incoming call, the music is playing on.
I forgot: any advice for this?
Yes, use the app offline / in flight mode! 
(this is my advice for my users, so they will not be disturbed while relaxing)