Here is a snap example of using the CSAudioPlayer in order to record sound of what you hear (WASAPI Loopback) to MP3 using C#

CSAudioRecorder-Snap-Example.cs

//The mode of the recording process. This can be WasapiLoopbackCapture(default), 
//WasapiCapture or LineIn
audioRecorder1.Mode = CSAudioRecorder.Mode.WasapiLoopbackCapture;

//The format of the destination file, this can be AAC, ACM(WAV), 
//APE, MP2, MP3, OGG, WAV(PCM), and WMA.
audioRecorder1.Format = CSAudioRecorder.Format.MP3;

//Set the destination file
audioRecorder1.FileName = lblDestinationFile.Text;

//On progress event
audioRecorder1.RecordProgress += (s, e) =>
{
    lblSizeIn.Text = audioRecorder1.TotalSizeIn; 
    // Or audioRecorder1.SizeSuffix(e.Bytes);
    
    lblTimeIn.Text = audioRecorder1.TotalTimeIn; 
    // Or TimeSpan.FromSeconds(e.Seconds).ToString();
};

//When record has done
audioRecorder1.RecordDone += (s) =>
{
    Console.WriteLine("\nDone.");
    done = true;
};

//Start to record
audioRecorder1.Start();