FFMPEG: Transparent background with 'showwaves' or 'showspectrum'
The default codec for MP4 is x264 which does not support alpha channel. Use PNG.

ffmpeg -i music.mp3 -filter_complex \
"[0:a]showwaves=s=1920x1080:mode=line:colors=white:draw=full,format=rgbap[v]" \
-map "[v]" -map 0:a -c:v png output.mp4;