To do this you will need a software synthesizer, which can convert from MIDI notes into PCM audio. Some software synthesizers make use of soundfonts as you mention, others use samples, and others generate sounds using oscillators or physical modelling.
NAudio does not include a software synthesizer, and building one is not a trivial task at all (will require knowledge of filters, resampling, oscillators, DSP etc). You might find that the best approach is to use NAudio in combination with VST.net. This would allow you to load any VST software instrument (of which there are thousands, both commerical and free), pass your MIDI into that, and use NAudio to write the resulting sound to a file.
Having said that, if all you need is a simple drum sequencer, the NAudio WPF demo does include an example of playing back drum samples.