title: Notification sound in lock-screen
body: <p>I'm developing my first android app, but a problem is coming.
  I have setted a notification sound with Firebase. My app currently works, it play the sound only when app is opened on the screen... If the screen is locked, I get the notification on my screen, but without playing the sound... how can I resolve it?</p>
  
  <p>Here's a part of the code:
  <a href="https://imghost.io/images/2017/09/19/Cattura.png" rel="nofollow noreferrer">https://imghost.io/images/2017/09/19/Cattura.png</a></p>