SongEncoder.cs

using System;
using System.IO;
using System.Collections.Generic;
using System.Diagnostics;

namespace SongEncoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //Encode all FLACs to Opus, copy jackets and retain folder structure.
            string startPath = @"D:\Music\";

            //Where to encode/copy
            string destinationPath = @"D\MusicEncoded\";


            //recursively encode, copy files and recreate folder structure
            void Do(string sDir)
            {
                foreach (string d in Directory.GetDirectories(sDir))
                {   
                    Directory.CreateDirectory(d.Replace("Music", "MusicEncoded"));
                    foreach (string file in Directory.GetFiles(d))
                    {
                        if (Path.GetExtension(file) == ".ogg" || Path.GetExtension(file) == ".mp3")
                        {
                            string newFile = d.Replace("Music", "MusicEncoded") + "\\" + Path.GetFileNameWithoutExtension(file);
                            var testString = String.Format("-i \"{0}\" -vn -acodec libopus -b:a 320K -vbr on -compression_level 10 \"{1}.opus\"", file, newFile);
                            DoEncode(file, testString);
                        }
                        else {
                            File.Copy(file, file.Replace("Music", "MusicEncoded"));
                        }
                    }
                    Do(d);
                }
            }
            
            Do(startPath);

            void DoEncode(string path, string encodeArgs)
            {

                var encodingProcess = new Process
                {
                    StartInfo = new ProcessStartInfo
                    {
                        //Change this to your ffmpeg install location
                        FileName = @"C:\ffmpeg\bin\ffmpeg.exe",
                        Arguments = encodeArgs,
                        UseShellExecute = false,
                        RedirectStandardOutput = true,
                        RedirectStandardError = true,
                        CreateNoWindow = false
                    }
                };
                
                Console.WriteLine(encodingProcess.StartInfo.Arguments);
                encodingProcess.EnableRaisingEvents = true;
                encodingProcess.ErrorDataReceived += (s, e) => Console.WriteLine($@"{e.Data}");
                encodingProcess.Start();
                encodingProcess.BeginOutputReadLine();
                encodingProcess.BeginErrorReadLine();
                encodingProcess.WaitForExit();
            }
        }
    }
}