Custom sound for Android Push Notification not working (FCM)

I have push notifications working using FCM from a cloud function. This works for both iOS and Android and displays the appropriate icon and plays a custom sound on iOS.

All is working except the custom sound for Android, it simply plays the default sound.

I have created a folder and added my sound file to it as follows: android\app\src\main\res\raw\mp3_example.mp3

This mp3 is 27s long. I have also tried a .wav and .aiff.

I read that I may have to create a push notification channel for later versions of Android so it could be related to this. I tried creating a channel and using the channelID from the cloud function and it works but there is no sound just a vibration.

The test device is a Moto G6 running Android 8. I am using: FCM Firebase Cloud Functions Ionic 4 Capacitor https://github.com/stewwan/capacitor-fcm

Cloud Function:

const notification: admin.messaging.Notification = {
      title: title,
      body: body
  }

  const message: admin.messaging.Message = {
    notification,
    topic: 'QMTBC',
    android:{
      notification:{
        sound: 'mp3_example.mp3',
        icon: 'push_logo',
        color: '#000000'
      }
    },
    apns:{
        payload:{
          aps: {
            sound: 'gears-short.wav'
          }
        }
    }
  }

  return admin.messaging().send(message)

app.component.ts

import { FCM } from 'capacitor-fcm';

const fcm = new FCM();
const { PushNotifications } = Plugins;

initializeApp() {
    this.platform.ready().then(() => {

      PushNotifications.register();

      PushNotifications.addListener('registration', (token: PushNotificationToken) => {
        console.log('token ' + token.value);
        fcm
        .subscribeTo({ topic: 'QMTBC' })
        .then(r => console.log(`subscribed to topic`))
        .catch(err => console.log(err));        
      });

      PushNotifications.addListener('registrationError', (error: any) => {
        console.log('error on register ' + JSON.stringify(error));
      });

      PushNotifications.addListener('pushNotificationReceived', (notification: PushNotification) => {
        console.log('notification ' + JSON.stringify(notification));
        this.pushNotificationService.notifications.push(notification);
      });

      PushNotifications.addListener('pushNotificationActionPerformed', (notification: PushNotificationActionPerformed) => {
        console.log('notification ' + JSON.stringify(notification));
        this.pushNotificationService.notifications.push(notification);
      });

      fcm.getToken()
        .then(r => console.log(`Token ${r.token}`))
        .catch(err => console.log(err));
    });
  }

UPDATE:

I tried creating a channel as follows. If I use the channel I just get the default sound. If I specific no channel or one that does not exist I also get the default sound (default channel).

cloud function:

const message: admin.messaging.Message = {
    notification,
    topic: 'QMTBC',
    android:{
      notification:{
        sound: 'punch.mp3',
        icon: 'push_logo',
        color: '#000000',
        channelId: 'QMTBC'
      }
    }

app.component.ts

const channel: PushNotificationChannel = {
          description: 'QMTBC',
          id : 'QMTBC',
          importance: 5,
          name : 'QMTBC'
        };

        PushNotifications.createChannel(channel).then(channelResult => {
          console.log(channelResult);
          console.log('Channel created');
          // PushNotifications.listChannels().then(channels => {
          //   console.log('Channels');
          //   console.log(channels);
          // });
        }, err => {
          console.log('Error Creating channel');
          console.log(err);
        });
      });

UPDATE 2:

I can see the channel I have created for the app on my device and it says the sound is default. I can manually change it to another inbuilt android sound and this works. But I still can’t use my custom sound.

Codes and updates using channels on stack overflow: