AudioServicesPlaySystemSound sound volume dependency - ios

Question

I'm making iOS keyboard extension.
To play click sound, I used AudioServicesPlaySystemSound.
However some users reported that the click sound sometimes depends on 'bell sound volume(bell icon)' and sometimes 'normal sound volume(speaker icon)'
I tested on Apple memo app and found there are cases about inconstant dependency.
Here is my code to init
func initTypeSound(soundIndex: Int) {
let bundle = NSBundle.mainBundle()
for var i = 0 ; i < MAX_TYPE_SOUND_COUNT ; i++ {
if let url = bundle.URLForResource(NSString(format: "type%d_%d", soundIndex, i) as String, withExtension: "wav") {
// file exist
var soundID : SystemSoundID = 0
AudioServicesCreateSystemSoundID(url, &soundID)
mTypeSoundIDs.insert(soundID, atIndex: i)
} else {
// no file
}
}
}
and code to play
func play(soundType: KKSoundType) {
if (!mHasPermission || !mIsSound) {
return
}
var session = AVAudioSession.sharedInstance()
let systemVolumn = session.outputVolume
if (systemVolumn == 0) {
return
}
var soundId : SystemSoundID!
switch (soundType) {
case .Type:
let rand = Int(arc4random_uniform(UInt32(mTypeSoundIDs.count)));
soundId = mTypeSoundIDs[rand];
break
case .Space:
soundId = mSpaceSoundID;
break
default:
return
}
if mIsSound {
AudioServicesPlaySystemSound(soundId)
}
}