Custom push notification sound not working in Oreo background

In oreo you have to set sound with channal

Please change your code with this

 if(Build.VERSION.SDK_INT>=Build.VERSION_CODES.O)
    {
        final Uri alarmSound = Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE
                + "://" + this.getPackageName() + "/raw/notification");
        AudioAttributes attributes = new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_NOTIFICATION)
                .build();
        NotificationChannel channel = new NotificationChannel("MyNotification","MyNotification", NotificationManager.IMPORTANCE_DEFAULT);
        channel.setSound(alarmSound,attributes);
        NotificationManager mgr =getSystemService(NotificationManager.class);
        mgr.createNotificationChannel(channel);

    }

it will work either your app is in background/foreground

Notification in Android oreo needs to be registered under the channel otherwise you will not able to generate a notification.

Check out the following article might help you

Click here

package com.example.notification;

import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.ContentResolver;
import android.content.Context;
import android.content.Intent;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.media.AudioAttributes;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;

import androidx.core.app.NotificationCompat;

import android.util.Log;

import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;


public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = "MyFirebaseMsgService";
    NotificationManager  mNotificationManager;
    NotificationCompat.Builder  mBuilder;
    Context mContext;

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {

        Log.d(TAG, "From: " + remoteMessage.getFrom());

        if (remoteMessage.getData().size() > 0) {
            Log.d(TAG, "Message data payload: " + remoteMessage.getData());


        }

        if (remoteMessage.getNotification() != null) {
            Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
        }
        sendNotification(remoteMessage.getNotification().getBody());

    }

    @Override
    public void onNewToken(String token) {
        Log.d(TAG, "Refreshed token: " + token);
        sendRegistrationToServer(token);
    }

    private void handleNow() {
        Log.d(TAG, "Short lived task is done.");
    }


    private void sendRegistrationToServer(String token) {
        // TODO: Implement this method to send token to your app server.
    }


    private void sendNotification(String message) {
        Intent resultIntent=new Intent(getApplicationContext(),MainActivity.class);
        PendingIntent resultPendingIntent = PendingIntent.getActivity(getApplicationContext(),
                0 /* Request code */, resultIntent,
                PendingIntent.FLAG_UPDATE_CURRENT);

//        Uri uri = Uri.parse("android.resource://" + getApplicationContext()
//                .getPackageName() + "/" + R.raw.mix);
        Uri soundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_ALARM);

        mBuilder = new NotificationCompat.Builder(getApplicationContext());
        mBuilder.setSmallIcon(R.drawable.ic_launcher_background);
        mBuilder.setLargeIcon(BitmapFactory.decodeResource(getResources(),
                R.drawable.ic_launcher_background));
        mBuilder.setContentTitle(message)
                .setContentText(message)
                .setSound(soundUri)
                .setLights(100,200,300)
                .setAutoCancel(false)
                .setVibrate(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400})
                .setColor(Color.GREEN)
                .setStyle(new NotificationCompat.BigTextStyle())
                .setContentIntent(resultPendingIntent)
                .setOnlyAlertOnce(true);

        mNotificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
            int importance = NotificationManager.IMPORTANCE_HIGH;
            NotificationChannel notificationChannel = new NotificationChannel("541201134433333", "MYGET", importance);
            notificationChannel.enableLights(true);
            notificationChannel.setLightColor(Color.RED);
            notificationChannel.enableVibration(true);
            notificationChannel.setShowBadge(true);
            notificationChannel.setVibrationPattern(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});
//            notificationChannel.s

            if (soundUri != null) {
                AudioAttributes audioAttributes = new AudioAttributes.Builder()
                        .setContentType(AudioAttributes.CONTENT_TYPE_SONIFICATION)
                        .setUsage(AudioAttributes.USAGE_ALARM)
                        .build();
                notificationChannel.setSound(soundUri, audioAttributes);
            }

            assert mNotificationManager != null;
            mBuilder.setChannelId("541201134433333");
            mNotificationManager.createNotificationChannel(notificationChannel);

        }

        assert mNotificationManager != null;
        mNotificationManager.notify(0 /* Request Code */, mBuilder.build());
    }
}