This sample applications shows how you can generate a sine wave with portamento in NAudio. For performance it uses a wavetable to store the sine wave, allowing this technique to be easily tried with other waveforms.
The SineWaveProvider class implements the tone generator. The Frequency property selects the desired frequency. Volume should be set between 0.0 (silence) and 1.0 (full volume). I recommend setting the volume very low before trying this out. PortamentoTime adjusts the time taken to reach the target frequency.