This is 100% reliable but yields exactly what you hear—including system sounds or compression artifacts. It is a last resort.
ffmpeg -i input.mp4 -vn -acodec pcm_s16le -ar 44100 output.wav niconico to wav
The "story" of Nico Nico Douga Monogatari.wav (ニコニコ動画物語.wav), often translated as "The Tale of Nico Nico Douga.wav," This is 100% reliable but yields exactly what