Custom push notification sound in android build doesn't work

I am building a Unity android application that implements GCM push notifications. The push notification is arriving fine but the custom sound won't seem to play. I have placed the custom sound file in the directory Assets/Plugins/Android/res/raw/notification_sound.wav and when I built the app and sent a push the default device notification sound played instead of my custom one. I am sending the GCM request via parse cloud code.

The GCM data object:

 {
   method: 'POST',
   url: 'https://gcm-http.googleapis.com/gcm/send',
   headers: {
     'Authorization': authorizationKey,
     'Content-Type': 'application/json'
   },
   body: {
     "registration_ids" : deviceTokens,
     "priority" : "high",
     "data" : {
         "ticker": "Game invitation",
         "content_text": senderName + " has invited you to a private game of Checkers!",
         "content_title": "Game invitation from " + senderName,
         "sound": "notification_sound"
     }
   }
 }

I have also tried using "sound":"notification_sound.wav" without success.

The device is running Android version 6.0.1 and the build was on Unity 5.5.0f3.

Thanks in advance for anyone willing to help :)

(Link to stackoverflow question: https://stackoverflow.com/questions/43660579/custom-push-notification-sound-in-unity-android-build-doesnt-work)