Exvel

using System;
using NAudio.Wave;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var waveIn = new WaveInEvent();
            waveIn.DataAvailable += WaveOnDataAvailable;
            waveIn.WaveFormat = new WaveFormat(8000, 1);
            waveIn.StartRecording();

            Console.ReadLine();
        }

        private static void WaveOnDataAvailable(object sender, WaveInEventArgs e)
        {
            for (int index = 0; index < e.BytesRecorded; index += 2)
            {
                short sample = (short)((e.Buffer[index + 1] << 8) | e.Buffer[index + 0]);

                float amplitude = sample / 32768f;
                float level = Math.Abs(amplitude); // от 0 до 1
                
                Console.WriteLine("Уровень: {0}%.", level * 100);
            }
        }
    }
}