// List of files from directory, sorted by *.wav type.
string[] filePaths = Directory.GetFiles(@"F:\Tankat\Music", "*.wav",
SearchOption.AllDirectories);
// Random number from 0 to the amount of files you have
Random rnd = new Random(Guid.NewGuid().GetHashCode());
int choices = rnd.Next(filePaths.Length);
// Create a new player with a random filepath from the array
SoundPlayer player = new SoundPlayer(filePaths[choices]);
player.Play();