Specify AudioSource = as_UseExternalAudio and specify the index of the audio device as follows, e.g.:
Videograbber.VideoSource = vs_IPCamera;
VideoGrabber.AudioSource = as_UseExternalAudio;
// pass to AudioDeviceIndex the name of the audio input as it appears in the “audio devices” list
// of MainDemo.exe or in the VideoGrabber.AudioDevices() list
Videograbber.AudioDevice = Videograbber.AudioDeviceIndex (“Microphone (HD Webcam C525)”);
VideoGrabber.AudioRecording = true;
Videograbber.IPCameraURL = “rtsp://…
Videograbber.RecordingMethod = rm_MP4;
Videograbber.StoragePath = “c:\myrecordings”;
// if RecordingFileName is not specified the file names are generated automatically
Videograbber.RecordingFileName = “c:\myrecordings\thisrecording.mp4”;
Videograbber.StartRecording();