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.

Requires ladspa-sdk and may use jack-audio-connection-kit.
Both are available from SlackBuilds.org