How to build screen recorder winform GUI app which also records the sound of system at the same time

using System;
using System.Diagnostics;
using System.Drawing;
using System.Windows.Forms;
using NAudio.Wave;
using SharpAvi;
using SharpAvi.Codecs;
using SharpAvi.Output;

namespace ScreenAudioRecorder
{
    public partial class Form1 : Form
    {
        private Stopwatch stopwatch;
        private string outputFileName;
        private WasapiLoopbackCapture audioCapture;
        private AviWriter aviWriter;
        private IAviVideoStream videoStream;
        private IAviAudioStream audioStream;
        private System.Windows.Forms.Timer screenCaptureTimer; // Specify the correct Timer
        private int frameRate = 10; // Frame rate for video

        public Form1()
        {
            InitializeComponent();
            stopwatch = new Stopwatch();
            screenCaptureTimer = new System.Windows.Forms.Timer(); // Specify the correct Timer
            screenCaptureTimer.Interval = 1000 / frameRate;
            screenCaptureTimer.Tick += CaptureScreenFrame;
        }

        private void OnFormLoad(object sender, EventArgs e)
        {
            stopwatch = new Stopwatch();
        }

        private void BtnRecord_Clicked(object sender, EventArgs e)
        {
            var dialog = new SaveFileDialog
            {
                Filter = "AVI files | *.avi"
            };

            if (dialog.ShowDialog() != DialogResult.OK)
            {
                return;
            }

            outputFileName = dialog.FileName;
            btn_record.Enabled = false;
            btn_stop.Enabled = true;

            aviWriter = new AviWriter(outputFileName)
            {
                FramesPerSecond = frameRate,
                EmitIndex1 = true
            };

            var screenBounds = Screen.PrimaryScreen.Bounds;
            videoStream = aviWriter.AddVideoStream();
            videoStream.Width = screenBounds.Width;
            videoStream.Height = screenBounds.Height;
            videoStream.Codec = CodecIds.Uncompressed;
            videoStream.BitsPerPixel = BitsPerPixel.Bpp24;

            audioCapture = new WasapiLoopbackCapture();
            var audioFormat = audioCapture.WaveFormat;
            audioStream = aviWriter.AddAudioStream(audioFormat.SampleRate, audioFormat.Channels, (int)audioFormat.BitsPerSample);
            audioStream.Name = "System Audio";

            audioCapture.DataAvailable += (s, a) =>
            {
                audioStream.WriteBlock(a.Buffer, 0, a.BytesRecorded);
            };

            audioCapture.StartRecording();
            screenCaptureTimer.Start();
            stopwatch.Start();
        }

        private void CaptureScreenFrame(object sender, EventArgs e)
        {
            var screenBounds = Screen.PrimaryScreen.Bounds;
            using (var bitmap = new Bitmap(screenBounds.Width, screenBounds.Height))
            using (var graphics = Graphics.FromImage(bitmap))
            {
                graphics.CopyFromScreen(0, 0, 0, 0, screenBounds.Size);
                var bits = bitmap.GetFrame();
                videoStream.WriteFrame(true, bits, 0, bits.Length);
            }
        }

        private void BtnStop_Clicked(object sender, EventArgs e)
        {
            btn_record.Enabled = true;
            btn_stop.Enabled = false;
            screenCaptureTimer.Stop();
            audioCapture.StopRecording();
            stopwatch.Stop();

            aviWriter.Close();
            audioCapture.Dispose();

            stopwatch.Reset();
        }

        private void TimerTick(object sender, EventArgs e)
        {
            lbl_Timer.Text = string.Format("{0:hh\\:mm\\:ss}", stopwatch.Elapsed);
        }
    }

    public static class BitmapExtensions
    {
        public static byte[] GetFrame(this Bitmap bitmap)
        {
            var bits = new byte[bitmap.Width * bitmap.Height * 3];
            var bitmapData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
            System.Runtime.InteropServices.Marshal.Copy(bitmapData.Scan0, bits, 0, bits.Length);
            bitmap.UnlockBits(bitmapData);
            return bits;
        }
    }
}