The sound font used by the system is always "big_synth.sy". You can replace the default General Midi sound font with a better version by linking "big_synth.sy" to your alternative sound font. One example of a better quality (and much bigger) sound font can be downloaded here (MIT license):

<http://ftp.de.debian.org/debian/pool/main/f/fluid-soundfont/fluid-soundfont_3.1.orig.tar.gz>
