Although the best sound quality is obtained by connecting JJazzLab to a good quality hardware synthesizer or a virtual instrument, JJazzLab can also render music using soundfonts (.sf2 files).
By default JJazzLab is using the Java Internal Synth (called Gervill) which render music using a basic soundfont. This does not produce high-quality sounds, especially on Linux.
To improve this you can load any GM-compatible .sf2 file using the menu Tools/Options/Midi, as indicated below.
See the Resources page for download links to GM-compatible .sf2 files.