Sound doesn't play on receiving push notification in ionic app

Question

to initialize push notifications in ionic app for iOS and android i add following code

const options: PushOptions = {
   android: {
   alert: 'true',
   badge: true,
   sound: 'true'
   },
   ios: {
       alert: 'true',
       badge: true,
       sound: 'true'
   },
   windows: {},
   browser: {
       pushServiceURL: 'http://push.api.phonegap.com/v1/push'
   }
};

const pushObject: PushObject = this.push.init(options);