Still quite strange. Fluidsynth is running fine in any game I’ve thrown it at so far, even when used as MT32 replacement, though that can sound rather horrible. Maybe (unless you haven’t already) copy the sf2 file to a standard directory, like /sdcard/Downloads, and see if that will help.
Because if you’re getting MT32 sound, MIDI output is definitely working on your device. No synth music output rather indicates that Dosbox can’t find/access the soundfont file.