For example, CoreAudio can use "SoundFonts" files. SoundFonts are used by sequencers, software synthetizers and even sound cards. They are files containing samples and descriptions to play a range of instruments. SoundFonts are, for example, shipped with SoundBlaster cards for PC.
In fact, when you play a MIDI file with QuickTime, or use an application that uses "QTMA" (QuickTime Musical Architecture), the sound of the instruments is generated by CoreAudio, using a default (and low end) SoundFonts provided by Roland. However, there *is* a way to replace the default SoundFont QT and CoreAudio will use.
Read the rest of the article for the how-to...
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 comparison, the default Roland SoundFont is 1.5Mb in size.
Once you've found a .sf2 file, copy it to ~/Library/Audio/Sounds/Banks. Then open the "System Preferences", select "QuickTime" and "Music", and select the SoundFont name from the list and click "Make Default". There you are! Now double click a MIDI file and play it in QuickTime Player, and it will use the SoundFont you downloaded!
Notes:
- QuickTime 6 preview includes a much better instrument set than before. However, it's still far from 'rich'.
- Some instruments from the SoundFonts you download might not work; the implementation of SoundFont playback appears to be unfinished in OSX 10.1.X.
[Editor's note: I have not tried this hint myself, but would be interested in hearing other's experiences with using sound fonts. For those more experienced in the audio world, are there standard locations where sound fonts and/or MIDI files can be legally downloaded? I know almost nothing about audio on the Mac beyond using iTunes to rip and play my CD collection!]

