Should work out of the box by using Magic Dosbox’s UI setup. I did the exact same steps as you: Midi:synth, select sf2 file (General_User_GS_Fluidsynth_v1.47.sf2), configured DOTT to use Gen MIDI Port 330, and it’s working.
That said, let’s look for the reason why it isn’t working for you.
• First off, please make sure there are no spaces in the file name of your .sf2 file. I think I recall there were some issues with that in Dosbox in general.
• Next, try the sound font I mentioned above. It’s free, and sounds really good (http://www.schristiancollins.com/generaluser.php).
• If it still doesn’t work, try it with another game, to see if it’s an isolated issue with your DOTT copy.
Good luck! ??