Soundfonts are banks of sounds that you can load into the MIDI synth memory. If you're using a Creative Labs soundcard (for example the SB Live) you can load the soundfonts banks with the soundcard's built-in software. When you then play a MIDI track with n-Track the MIDI output will have the sounds of the soundfonts bank. You can also load more than one soundfont bank and switch between them with the bank setting on the MIDI track's property page.
Alternatively there are VST/DX instruments synths that support soundfonts.
See the Plugins page.
n-Track Studio 10 