How to add custom notification sound in Android

Firstly make the folder in Resource (res) name it raw and put the file (YOUR_SOUND_FILE.MP3) in it and than use below lines of code for custom sound

   NotificationManager notificationManager = (NotificationManager) context
            .getSystemService(Context.NOTIFICATION_SERVICE);
    Notification notification = new Notification(icon, message, when);

    String title = context.getString(R.string.app_name);

    Intent notificationIntent = new Intent(context,
            SlidingMenuActivity.class);
    notificationIntent.putExtra("isInbox", true);
    // set intent so it does not start a new activity
    notificationIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP
            | Intent.FLAG_ACTIVITY_SINGLE_TOP);
    PendingIntent intent = PendingIntent.getActivity(context, 0,
            notificationIntent, 0);
    notification.setLatestEventInfo(context, title, message, intent);
    notification.flags |= Notification.FLAG_AUTO_CANCEL;

Use these lines of code for custom sound

 notification.sound =Uri.parse("android.resource://"+context.getPackageName()+"/"+R.raw.FILE_NAME);//Here is FILE_NAME is the name of file that you want to play


    // Vibrate if vibrate is enabled
    notification.defaults |= Notification.DEFAULT_VIBRATE;
    notificationManager.notify(0, notification);

EDIT UPDATE

For the Oreo and higher , you need to check the SDK_VERSION and use the setSound method of the NotificationChannel

Uri sound = Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE + "://" + context.getPackageName() + "/" + R.raw.FILE_NAME);  //Here is FILE_NAME is the name of file that you want to play

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {

            NotificationChannel mChannel = new NotificationChannel("YOUR_CHANNEL_ID",
                "YOUR CHANNEL NAME",
                NotificationManager.IMPORTANCE_DEFAULT)

            AudioAttributes attributes = new AudioAttributes.Builder()
                    .setUsage(AudioAttributes.USAGE_NOTIFICATION)
                    .build();

            NotificationChannel mChannel = new NotificationChannel(CHANNEL_ID, 
                    context.getString(R.string.app_name),
                    NotificationManager.IMPORTANCE_HIGH);

            // Configure the notification channel.
            mChannel.setDescription(msg);
            mChannel.enableLights(true);
            mChannel.enableVibration(true);
            mChannel.setSound(sound, attributes); // This is IMPORTANT


            if (mNotificationManager != null)
                mNotificationManager.createNotificationChannel(mChannel);
        }

Intent i = new Intent(this, MainActivity.class);
PendingIntent pi = PendingIntent.getActivity(this, 0, i, PendingIntent.FLAG_CANCEL_CURRENT);

NotificationCompat.Builder builder = new NotificationCompat.Builder(this)
    .setContentTitle("I want food")
    .setContentText(notificationcontent)
    .setSmallIcon(R.drawable.ic_launcher)
    .setContentIntent(pi)
    .setAutoCancel(true)
    .setDefaults(Notification.FLAG_ONLY_ALERT_ONCE);
NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
MediaPlayer mp= MediaPlayer.create(contexto, R.raw.your_sound);
mp.start();
manager.notify(73195, builder.build());