C# Events Tutorial

In the class that throws the events:

First declare a delegate:

private delegate void AudioIsPlayingHandler(MediaElement elem);

Then declare an event:

private event AudioIsPlaingHandler OnAudioIsPlaying;

Somewhere, the class throws the event:

if (OnAudioIsPlaying != null)
    OnAudioIsPlaying(elem);   // Raise the event and pass some data

Now, in a class that has an instance of the previous class:

First, attach to the event in obj A, an instance of the previous class:

A.OnAudioIsPlaying += new System.EventHandler(A_OnAudioIsPlaying);

Then, in the handler function:

private void A_OnAudioIsPlaying(MediaElement elem)
{
   // Do something with the data received
}

Finally, check this out for a lot more details:
Visual Studio Developer Center - Events Tutorial