converting all the mp4 audio files in a folder to mp3 using ffmpeg

FileInfo[] Files = d.GetFiles("*.mp4").Union(d.GetFiles("*.<any other file extension>")).ToArray(); 

foreach (FileInfo file in Files)
            {
               // str = str + ", " + file.Name;
                  builder.Append(Path.GetFileNameWithoutExtension(file.Name));

ProcessStartInfo startInfo = new ProcessStartInfo();
       startInfo.CreateNoWindow = false;
       startInfo.UseShellExecute = false;
       startInfo.FileName = "ffmpeg.exe";
       startInfo.WindowStyle = ProcessWindowStyle.Hidden;
       startInfo.RedirectStandardOutput = !string.IsNullOrEmpty("test");
     startInfo.Arguments = "-i " + filename + ".mp4 -q:a 0 -map a " + filename + ".mp3";

}

One method is a bash for loop.

For converting only .mp4 files:

mkdir outputs
for f in *.mp4; do ffmpeg -i "$f" -c:a libmp3lame "outputs/${f%.mp4}.mp3"; done

For converting .m4a, .mov, and .flac:

mkdir outputs
for f in *.{m4a,mov,flac}; do ffmpeg -i "$f" -c:a libmp3lame "outputs/${f%.*}.mp3"; done

For converting anything use the "*" wildcard:

mkdir outputs
for f in *; do ffmpeg -i "$f" -c:a libmp3lame "outputs/${f%.*}.mp3"; done