Generate wav file with Amazon Polly and .NET C#

SpeechGenerator.cs

using System;
using System.Collections.Generic;
using System.IO;
using Amazon.Polly; //nuget AWSSDK.Core AWSSDK.Polly
using Amazon.Polly.Model;
using NAudio.Wave;  //nuget NAudio

public class SpeechGenerator
{
  public string awsAccessKey = "";
  public string awsAccessSecret = "";
  
  public async void GenerateVoiceWavFromText(string textMessage, string path, string fileName)
  {
    if (string.IsNullOrEmpty(textMessage))
    {
        throw new Exception("textMessage can't be empty");
    }

    if (string.IsNullOrEmpty(path))
    {
        throw new Exception("path can't be empty");
    }

    if (string.IsNullOrEmpty(fileName))
    {
        throw new Exception("fileName can't be empty");
    }

    if (!Directory.Exists(path))
    {
        Directory.CreateDirectory(path);
    }

    string outputFileName = Path.Combine(path, fileName);

    var config = new AmazonPollyConfig();
    
    var client = new AmazonPollyClient(this.awsAccessKey, this.awsAccessSecret, config);

    var synthesizeSpeechRequest = new SynthesizeSpeechRequest()
    {
      OutputFormat = OutputFormat.Pcm,
      VoiceId = VoiceId.Brian,
      Text = textMessage,
    };

    using (var outputStream = new FileStream(outputFileName, FileMode.Create, FileAccess.Write))
    {
      var synthesizeSpeechResponse = await client.SynthesizeSpeechAsync(synthesizeSpeechRequest);
      byte[] buffer = new byte[2 * 1024];
      int readBytes;

      var inputStream = synthesizeSpeechResponse.AudioStream;
      while ((readBytes = inputStream.Read(buffer, 0, 2 * 1024)) > 0)
      {
         outputStream.Write(buffer, 0, readBytes);
      }

      outputStream.Flush();
      outputStream.Close();
    }

    using (var readPcmStream = File.OpenRead(outputFileName))
    {
      using (var rawWaveStream = new RawSourceWaveStream(readPcmStream, new WaveFormat(16000, 1)))
      {
        var outpath = outputFileName + ".wav";
        WaveFileWriter.CreateWaveFile(outpath, rawWaveStream);

        rawWaveStream.Close();
      }

      readPcmStream.Close();
    }
  }
}