capture sound from mic and play it on speakers in c#

If you want to play the same file while it is capturing, you can achieve that using LEADTOOLS Multimedia toolkit controls and enabling Audio Preview.
The following few lines of code will do the job:
/*******************************************/
CaptureCtrl1.AudioDevices[0].Selected= 0;
CaptureCtrl1.VideoDevices[0].Selected= 0;
CaptureCtrl1.PreviewSource = CapturePreview.Audio;
CaptureCtrl1.TargetFormat = TargetFormatType.MP3;
CaptureCtrl1.TargetFile = @"C:\output.avi";
CaptureCtrl1.StartCapture(CaptureMode.VideoAndAudio);
/*******************************************/
Disclaimer: I work for the company that created this library.