Does the system produce sound (so you know sound drivers are working).
If yes, does fluidsynth produce sound output if you use it to play a midi file from a to terminal (you have to provide cli options for output device, eg ALSA, and soundfont, full path).
If yes, can you start fluidsynth in server mode and, optionally, specify the name (or perhaps use QSynth for this), or use #> ps -ef to see the fluidsynth process id, then check in Frescobaldi if that process ID/name shows in the midi preferences config?
It's flyidsynth that converts midi to sound. If I remember rightly all Frescobaldi does is pass the midi data to fluidsynth when it's running in server mode.
But you can probably play midi files with timiditu, playmidi, etc even if the above won't work.
Hopefully.