Push notifications sound not coming

I tried to use like below

I am using pyfcm to send notifications (https://github.com/olucurious/PyFCM/).

push_service = FCMNotification(api_key)
push_service.notify_multiple_device(registration_ids=registration_ids, message_title=message_title,
message_body=message_body, data_message=data_message, sound="Default")

I added sound="Default" as last parameter of that function. But mobile not making any sounds even though notification is coming.

This is a react native application but I think react native has nothing to do here because any mobile app doesnt even know when the notification comes in background.

Any idea how to fix this? Thanks