Downloader.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Threading.Tasks;
using System.Web;

namespace ConsoleYoutubeDownloader
{
    public class Downloader
    {
        private readonly string _ytPath;
        private readonly string _folderPath;

        public Downloader(string folderPath, string ytPath)
        {
            _folderPath = folderPath;
            _ytPath = GetYoutubeURI(ytPath);
        }

        public async Task Download(Action<string> progressAction)
        {
            var apiRequest = WebRequest.Create($"http://youtubeinmp3.com/fetch/?format=text&video={_ytPath}");
            Task<WebResponse> apiResponseTask = apiRequest.GetResponseAsync();
            progressAction($"API requested for {_ytPath}");
            string[] apiDetails;
            using (var apiResponse = await apiResponseTask)
            {
                using (StreamReader sr = new StreamReader(apiResponse.GetResponseStream()))
                {
                    var apiDetailsTask = sr.ReadToEndAsync();
                    progressAction($"Downloading API response for {_ytPath}");
                    apiDetails = (await apiDetailsTask).Split('\n');
                }
            }
            if (apiDetails.Length != 3) throw new InvalidDataException("YoutubeInMP3 response invalid.");
            var songRequest = WebRequest.Create(apiDetails[2]);
            Task<WebResponse> songResponseTask = apiRequest.GetResponseAsync();
            progressAction($"Song requested for {_ytPath}");
            using (var songResponse = (await songResponseTask).GetResponseStream())
            {
                string fileName = Path.ChangeExtension(string.Join("", apiDetails[0].Skip(7).Except(Path.GetInvalidFileNameChars())), ".mp3");
                Console.WriteLine(Path.Combine(_folderPath, fileName));
                using (FileStream songStream = File.Create(Path.Combine(_folderPath, fileName)))
                {
                    byte[] buffer = new byte[32*1024];
                    int read = songResponse.Read(buffer, 0, buffer.Length);
                    while (read > 0)
                    {
                        songStream.Write(buffer, 0, read);
                        read = songResponse.Read(buffer, 0, buffer.Length);
                    }
                }
            }
        }

        public static string GetYoutubeURI(string path)
        {
            Uri ytUri = new Uri(path);
            if (ytUri.Host == "youtu.be")
                return $"http://youtube.com/watch?v={ytUri.AbsolutePath.TrimStart('\\')}";
            if (ytUri.Host.Contains("youtube.com"))
                return $"http://youtube.com/watch?v={HttpUtility.ParseQueryString(ytUri.Query)["v"]}";
            throw new ArgumentException("Not a valid Youtube URI.");
        }
    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

namespace ConsoleYoutubeDownloader
{
    class Program
    {
        private static string _folderPath = "";
        static void Main(string[] args)
        {
            List<string> toDownload = new List<string>();
            if (args.Length > 0 && Directory.Exists(args[0]))
            {
                _folderPath = args[0];
                if (args.Length > 1) toDownload.AddRange(args.Skip(1));
            }
            else
            {
                Console.WriteLine("Enter folder path where you would like output .mp3 files to be saved:");
                for (;;)
                {
                    string folderTemp = Console.ReadLine();
                    if (!Directory.Exists(folderTemp ?? ""))
                    {
                        Console.WriteLine("Not a valid folder path, please enter another one.");
                        continue;
                    }
                    _folderPath = folderTemp;
                    break;
                }
            }
            List<Task> tasks = new List<Task>();
            if (toDownload.Any())
                tasks.AddRange(
                    toDownload.Select(
                        ytPath =>
                            new Downloader(_folderPath, ytPath).Download(Console.WriteLine)
                                .ContinueWith(t => Console.WriteLine("Completed download."))));
            if (tasks.Any()) Task.WaitAll(tasks.ToArray());
            for (;;)
            {
                Console.WriteLine("Enter any youtube videos you would like downloaded:");
                tasks.Add(
                    new Downloader(_folderPath, Console.ReadLine()).Download(Console.WriteLine)
                        .ContinueWith(t => Console.WriteLine("Completed download.")));
                if (tasks.Any()) Task.WaitAll(tasks.ToArray());
                foreach (Task t in tasks.ToArray()) if (t.IsCompleted) tasks.Remove(t);
            }
        }
    }
}