According to the Xamarin docs it seems to be pretty straightforward:
Create a new Xamarin.Android application named PlayAudio.
Add a sub folder named raw under Resources.
Add a file named test.mp3 under raw.
In the Activity, create a class variable for the MediaPlayer.
MediaPlayer _player;
In the OnCreate method, call MediaPlayer.Create, passing the context and the resource identifier for the mp3.
_player = MediaPlayer.Create(this, Resource.Raw.test);
Call the Start method of the MediaPlayer.
_player.Start();