Not able play multiple sounds

I have one long background sound. The other sound will occur when a new message come. But when the message sound start the background song is stopped with following error.

ERROR: [0x1a0cdb000] AVAudioSession.mm:692: -[AVAudioSession setActive:withOptions:error:]: Deactivating an audio session that has running I/O. All I/O should be stopped or paused prior to deactivating the audio session.

However this only happens when I run my application on my iPhone 6 iOS 9.3 (haven't tested any other hardware), but does not occur when I run the application in the iOS Simulator.

Any suggestions or tips are highly appreciated.
Thanks in advance