This prototype pipes audio stream to ffmpeg and get result stream back.

ffmpegPiping.csx

#r "nuget: YoutubeExplode, *"

using YoutubeExplode;
using YoutubeExplode.Models.MediaStreams;

const string FFmpegPath = "./ffmpeg.exe";
var youtubeClient = new YoutubeClient();
var mediaStreamInfo = await youtubeClient.GetVideoMediaStreamInfosAsync("uBg-nNJCnmM");
var audioStream = mediaStreamInfo.Audio.WithHighestBitrate();
var processStartInfo = new ProcessStartInfo
{
    FileName = Path.GetFullPath(FFmpegPath),
    Arguments = $"-i pipe:0 -vn -ar 44100 -ac 2 -ab 192k -f mp3 -",
    RedirectStandardInput = true,
    RedirectStandardOutput = true,
    UseShellExecute = false
};

using (var process = Process.Start(processStartInfo))
{
    using (var fs = new FileStream("output.mp3", FileMode.CreateNew))
    {
        var downloadTask = youtubeClient.DownloadMediaStreamAsync(audioStream, process.StandardInput.BaseStream);
        var outputTask = process.StandardOutput.BaseStream.CopyToAsync(fs);
        await downloadTask;
        process.StandardInput.Close();
        await outputTask;
        process.StandardOutput.Close();
    }
    process.WaitForExit();
}