React native firebase messaging notification sound not change in background

i am using @react-native-firebase/messaging for push notification in my application. and for custom notification sound i use @notifee/react-native this.

my current notification code is :

import messaging from '@react-native-firebase/messaging';
import notifee from '@notifee/react-native';

await notifee.createChannel({
      id: 'custom-sound',
      name: 'System Sound',
      sound: 'notification.mp3',
});

messaging().onMessage((remoteMessage) => {
      notifee.displayNotification({
        title: remoteMessage.notification.title,
        body: remoteMessage.notification.body,
        android: {
          channelId: 'custom-sound',
        },
      });
});

the above code works fine when the application is open on the screen so that my custom notification sound works perfectly.

but when the app is running in the background then the notification sound is the default (system default)

so how i can set the custom notification sound in all cases.