In my hybrid Cordova Android app targeting API 23+ I want to use a custom sound for notifications. To that end I have done the following
plugin.xml
Notification.Builder builder = new Notification.Builder(context);
builder.setContentTitle(mTitle);
builder.setContentText(mContentText);
builder.setSmallIcon(R.mipmap.ic_launcher);
builder.setSound(Settings.System.DEFAULT_NOTIFICATION_URI);
builder.setVibrate(new long[] { 1000, 1000, 1000, 1000, 1000 });
builder.setDefaults(Notification.DEFAULT_ALL);
Use this to work with sound, I hope it will solve your problem, Cheers!