No... Is the dafault sound. I checked. The problem is not the notification but the are not running on background. I checked at my friends phone and the send message only gets one "check". It means that my phone don't even received the message.
But it sometimes works and sometimes don't.. I don't know why.. Its not related to network also.
If your friends see only one sign on messages, THAT's a network iussue.
You are making confusion: a background working app doesn't means you find already open in the backgroud. Consider your phone as a notebook: do you know how background apps are working? that's the same. App is working, NOT already opened.
If you enabled that in Background, and disabled keep private in lock screen in Notify settings, they should work. Then, make a soft reset.
If that's a network iussue, check internet APN settings, maybe you lost them during update