for /r %a in (*.flac *.m4a *.aac *.ogg) do ffmpeg -i "%a" -vn -acodec libmp3lame -q:a 0 "%~pa%~na.mp3"
Run the command while in your music folder and it will recurse through subfolders, converting all files found having the specified extensions into .mp3 format using LAME VBR v0. .mp3 files will be placed in the same directory where it found the .m4a/.flac/.aac/.ogg
Will need
ffmpeg installed and in your %PATH% environment.