FluidSynth is a software real-time synthesizer based on the
Soundfont 2 specifications. FluidSynth reads and handles MIDI
events from the MIDI input device. It is the software analogue
of a MIDI synthesizer. FluidSynth can also play midifiles
using a Soundfont.

If you want support for ladspa plugins you have to pass LADSPA=yes
to the script. This requires ladspa_sdk

Additionally fluidsynth may use libsndfile, lash, pulseaudio,
portaudio and jack-audio-connection-kit. All of which are available
from SlackBuilds.org