How to join 2 or more .WAV files together programmatically?

If you need get only byte array, to insert in database or somebody else. You may use memory stream:

        public static byte[] Concatenate(IEnumerable<byte[]> sourceData)
    {
        var buffer = new byte[1024 * 4];
        WaveFileWriter waveFileWriter = null;

        using (var output = new MemoryStream())
        {
            try
            {
                foreach (var binaryData in sourceData)
                {
                    using (var audioStream = new MemoryStream(binaryData))
                    {
                        using (WaveFileReader reader = new WaveFileReader(audioStream))
                        {
                            if (waveFileWriter == null)
                                waveFileWriter = new WaveFileWriter(output, reader.WaveFormat);
                            else
                                AssertWaveFormat(reader, waveFileWriter);

                            WaveStreamWrite(reader, waveFileWriter, buffer);
                        }
                    }
                }

                waveFileWriter.Flush();

                return output.ToArray();
            }
            finally
            {
                waveFileWriter?.Dispose();
            }
        }
    }

    private static void AssertWaveFormat(WaveFileReader reader, WaveFileWriter writer)
    {
        if (!reader.WaveFormat.Equals(writer.WaveFormat))
        {
            throw new InvalidOperationException("Can't concatenate WAV Files that don't share the same format");
        }
    }

    private static void WaveStreamWrite(WaveFileReader reader, WaveFileWriter writer, byte[] buffer)
    {
        int read;
        while ((read = reader.Read(buffer, 0, buffer.Length)) > 0)
        {
            writer.Write(buffer, 0, read);
        }
    }