Niconico streams audio in AAC or similar formats. Converting this to WAV ( PCM ) will not increase the quality above the source; it only prevents further loss of quality (unlike converting to MP3, which adds lossy compression).
Niconico requires login. Run: yt-dlp --username YOUR_EMAIL --password YOUR_PASSWORD https://www.nicovideo.jp/watch/SM12345678 niconico to wav
Command line snippet for forensic extraction (using yt-dlp + ffmpeg ): Niconico streams audio in AAC or similar formats