You need to set your app Capabilities Background Modes (Audio and AirPlay) and set your AVAudioSession category to AVAudioSessionCategoryPlayback and set it active
From Xcode 11.4 • Swift 5.2
do {
try AVAudioSession.sharedInstance().setCategory(.playback, mode: .default, options: [.mixWithOthers, .allowAirPlay])
print("Playback OK")
try AVAudioSession.sharedInstance().setActive(true)
print("Session is Active")
} catch {
print(error)
}