Hi @Vic ,
Forgive me I misunderstood your question, but I don't believe your notification message could (or should) contain the payload for the actual sound. The sound should be an asset in your app package.
I did see https://stackoverflow.com/a/43284214/10987310 which contains a code snippet of how to play a custom sound with a notification. If you include the name of the sound file in the payload and use it to make the resource Uri.
Regards,
Ryan