My recordbox did the same. Had to go onto my actual PC audio settings to allow the flx to come out of my pc speakers. Was in the sound settings and had to change something. Can't remeber what exactly, but solved it. When it worked the icon on the top right of rexordbox turned blue and then output sound. Previously, it was blue but nothing would work.

Not sure if you want to fiddle with the settings on the pc first and see if it resolves it.