`showspectrum=color=rainbow:legend=1’

Watch on youtube.com
doc

https://ffmpeg.org/ffmpeg-filters.html#showspectrum

#! /bin/sh
ifn="Air on the G String (from Orchestral Suite no. 3, BWV 1068).mp3"
ifnb="`basename \"${ifn}\" .mp3`"
pref="`basename $0 .sh`"

#
"/c/Program Files/ffmpeg-4.1-win64-shared/bin/ffmpeg" -y \
    -i "${ifn}" -filter_complex "
[0:a]showspectrum=s=1920x1080:color=rainbow:legend=1[v]
" -map '[v]' -map '0:a' -c:a copy \
  "${pref}_${ifnb}.mp4"