I posted the same issue on the DarkMX forum. I noticed the sound notifications also don't work on Fopnu. This isn't that big of a deal, since the chat bubble pop up option works, but it would be nice if the sound notifications worked as well. Not just for chat messages either. Sound notifications for all events don't work. Maybe it's some sort of bug perhaps? All of my other apps sound notifications work so I know it's nothing to do with my system sound settings.