Retrieve a list of soundfiles from the folder, then just pick a random number between 0 and list.Length-1 and select that file.
//Untested code, but should give you an idea.
string[] files = Directory.GetFiles("path");
Random rnd = new Random(Guid.NewGuid().GetHashCode());
int choice = rnd.Next(0, files.Length - 1);
string soundFile = files[choice];
System.Media.SoundPlayer player = new System.Media.SoundPlayer(soundFile);
player.Play();