Mac OS includes an highly optimized virtual instrument (DLSMusicDevice) compatible with both the industry standard DownLoadable Sounds (DLS) format and SoundFonts. The Mac synthesizer provides applications with high quality and low latency sample playback.
By default, the "QuickTime Music Synthesizer" SoundFont is used, but you can easily install and use other SoundFonts:
First, download a SoundFont (a .sf2 file). The SoundFont must be at least GM (General MIDI, with 128 instruments) or better GS (GM plus some Roland Extensions). You can find SoundFonts from 4Mb to 128+Mb in size, depending on the quality and richness of the instrument set. For example, the default Roland SoundFont is 1.5Mb in size.
Once you've found a .sf2 file, copy it to "~/Library/Audio/Sounds/Banks/", then restart this application, select the SoundFont name in the "Sound Bank" popup menu and save the document.
NB: Because of a bug in the Apple DLS synth view, the popup may display the first soundfont instead of the selected soundfont.