To convert m4p to mp3, you need an apple music converter because m4p is normally apple music songs. Try TunesKit, it is really nice because it is an all-round mighty DRM removal solution to convert DRM-protected Apple Music, iTunes M4P and audiobooks, Audible AA, AAX, as well as non-DRM audios to common MP3, AAC, M4A, M4B, WAV, FLAC for any device and player.