您可能已经使用默认声音创建了最初的频道。一旦创建了频道，就无法更改。您需要重新安装应用程序或使用新的频道ID创建频道。

我用RingtoneManager，它是为我工作.尝试thius代码：

NotificationCompat.Builder builder = new NotificationCompat.Builder(MainActivity.this);
    builder.setSmallIcon(android.R.drawable.ic_dialog_alert);
    Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.google.com/"));
    PendingIntent pendingIntent = PendingIntent.getActivity(MainActivity.this, 0, intent, 0);
    builder.setContentIntent(pendingIntent);
    builder.setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher));
    builder.setContentTitle("Notification title");
    builder.setContentText("Notification message.");
    builder.setSubText("Url link.");

    try {
        Uri notification = Uri.parse("android.resource://" + getPackageName() + "/" + R.raw.custom_ringtone);
        Ringtone r = RingtoneManager.getRingtone(getApplicationContext(), notification);
        r.play();
    } catch (Exception e) {
        e.printStackTrace();
    }

    NotificationManager notificationManager = (NotificationManager) this.getSystemService(NOTIFICATION_SERVICE);
    notificationManager.notify(1, builder.build());

在奥利奥中，您需要为此创建一个通道。https://developer.android.com/reference/android/app/NotificationChannel.html

默认声音将覆盖任何声音。
你需要把它放在你的代码中：

notification.defaults = Notification.DEFAULT_LIGHTS | Notification.DEFAULT_VIBRATE;

参考：
Android notifications

除了Faxriddin的答案之外，您还可以通过检查importance of the notification channel和are enabled notifications参数来确定何时应该关闭通知声音。

NotificationChannel channel = notificationManager.getNotificationChannel(NOTIFICATION_CHANNEL_ID);
if(notificationManager.areNotificationsEnabled() && channel.getImportance() != NotificationManager.IMPORTANCE_NONE) {
  try {
     Ringtone r = RingtoneManager.getRingtone(ctx, soundUri);
     r.play();
  } catch (Exception e) { }
}

我一直在开发具有类似功能的应用程序。正如帕维尔·纳多尔斯基所提到的，我们每次都要重新创建频道来改变铃声。为此我写了一个助手。希望它能帮助到别人。

@RequiresApi(Build.VERSION_CODES.O)
object ChannelHelper {
    // channel id for 8.0 OS version and higher
    private const val OREO_CHANNEL_ID = "com.ar.app.notifications"
    private const val CHANNEL_ID_PREF = "com.ar.app.notifications_prefs"

    @JvmStatic
    fun createChannel(context: Context, playSound: Boolean, isVibrated: Boolean, uri: Uri) {
        // establish name and importance of channel
        val name = context.getString(R.string.main_channel_name)
        val importance = if (playSound) {
            NotificationManager.IMPORTANCE_DEFAULT
        } else {
            NotificationManager.IMPORTANCE_LOW
        }

        // create channel
        val channelId = OREO_CHANNEL_ID + UUID.randomUUID().toString()
        saveChannelId(context, channelId)

        val channel = NotificationChannel(channelId, name, importance).apply {
            enableLights(true)
            lightColor = Color.GREEN

            lockscreenVisibility = Notification.VISIBILITY_PUBLIC

            // add vibration
            enableVibration(isVibrated)
            vibrationPattern = longArrayOf(0L, 300L, 300L, 300L)

            // add sound
            val attr = AudioAttributes.Builder()
                    .setUsage(AudioAttributes.USAGE_NOTIFICATION_RINGTONE)
                    .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
                    .build()

            setSound(uri, attr)
        }

        // register the channel in the system
        val notificationManager = context.getSystemService(NotificationManager::class.java)
        notificationManager.createNotificationChannel(channel)
    }

    @JvmStatic
    fun rebuildChannel(context: Context, playSound: Boolean, isVibrated: Boolean, uri: Uri) {
        val notificationManager = context.getSystemService(NOTIFICATION_SERVICE)
                as NotificationManager
        notificationManager.deleteNotificationChannel(
            getChannelId(context) ?: OREO_CHANNEL_ID
        )

        createChannel(context, playSound, isVibrated, uri)
    }

    @JvmStatic
    fun getChannel(context: Context): NotificationChannel? {
        val notificationManager = context.getSystemService(NOTIFICATION_SERVICE)
                as NotificationManager

        return notificationManager.getNotificationChannel(
                getChannelId(context) ?: OREO_CHANNEL_ID
        )
    }

    @JvmStatic
    fun isChannelAlreadyExist(context: Context) = getChannel(context) != null

    @JvmStatic
    fun getChannelId(context: Context) =
            PreferenceManager.getDefaultSharedPreferences(context)
                    .getString(CHANNEL_ID_PREF, OREO_CHANNEL_ID)

    private fun saveChannelId(context: Context, channelId: String) =
            PreferenceManager.getDefaultSharedPreferences(context)
                    .edit()
                    .putString(CHANNEL_ID_PREF, channelId)
                    .apply()
}