Main.axml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <Button
        android:id="@+id/start"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Start Recording" />
    <Button
        android:id="@+id/stop"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:enabled="false"
        android:text="Stop Recording" />
</LinearLayout>

MainActivity.cs

using Android.App;
using Android.Widget;
using Android.OS;
using System.IO;
using Android.Media;

namespace VoicRec2
{
    [Activity(Label = "VoicRec2", MainLauncher = true)]
    public class MainActivity : Activity
    {
        MediaRecorder _recorder;
        MediaPlayer _player;
        Button _start;
        Button _stop;
        string path;
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            SetContentView(Resource.Layout.Main);
            _start = FindViewById<Button>(Resource.Id.start);
            _stop = FindViewById<Button>(Resource.Id.stop);
            path = Path.Combine(this.GetExternalFilesDir(Android.OS.Environment.DirectoryMusic).AbsolutePath, "MyWav.wav");



            _start.Click += delegate {
                _stop.Enabled = !_stop.Enabled;
                _start.Enabled = !_start.Enabled;

                _recorder.SetAudioSource(AudioSource.Mic);
                _recorder.SetOutputFormat(OutputFormat.ThreeGpp);

                _recorder.SetAudioChannels(1);
                _recorder.SetAudioSamplingRate(16);
                _recorder.SetAudioEncodingBitRate(16000);
                
                _recorder.SetAudioEncoder((AudioEncoder) Encoding.Pcm16bit);

                _recorder.SetOutputFile(path);
                _recorder.Prepare();
                _recorder.Start();
            };


            _stop.Click += delegate {
                _stop.Enabled = !_stop.Enabled;

                _recorder.Stop();
                _recorder.Reset();

                _player.SetDataSource(path);
                _player.Prepare();
                _player.Start();
            };

        }
        protected override void OnResume()
        {
            base.OnResume();

            _recorder = new MediaRecorder();
            _player = new MediaPlayer();

            _player.Completion += (sender, e) => {
                _player.Reset();
                _start.Enabled = !_start.Enabled;
            };

        }
        public void onStart()
        {


        }
        protected override void OnPause()
        {
            base.OnPause();

            _player.Release();
            _recorder.Release();
            _player.Dispose();
            _recorder.Dispose();
            _player = null;
            _recorder = null;
        }
    }
}