Aloha.
I've verified that I qsynth produces sound when I use an Arturia keyboard as input. Also, qsynth shows events being received from FB but still no sound.
No sure what else to try. FB settings appear correct and, as I say, the green light on qsynth flashes indicating (I think) FB is sending it events. Don't know if it's possible to look at the events to see if they make sense.
Any suggestions are welcome.