C#

In the below code snippet, Ring() method defined in Classes "SmartPhone", "BasicPhone" etc is returning a custom datatype called "Sound". As far as I have read few topics, custom datatypes can be implemented using Classes, Structures etc.Normally, I have worked with methods returning in-built datatypes like int,string etc but have no idea about methods returning custom datatypes like "Sound" as per mentioned example. So, how should I write/define the Class "Sound" for the above program to work

If code execution calls "Phone.Ring()" statement in the foreach loop defined Main() method , let's say a message "MP3 music is playing" for Smartphone, "Polyphonic music is playing" for SemiSmartPhone, "8BitMusic is playing" for BasicPhone to be displayed on the console screen. 
Let me know if you need more details.To be honest, had posted in other forums, but couldn't get an answer yet.As far as I remember, this code example has been picked from codeproject.com I believe.  Thanks

class Sound
{
    ???// What should the implementation of this class be for the program    
       //to work
}
interface IRingable
{
    public Sound Ring();
}
class SmartPhone : IRingable
{
    public Sound Ring()
    {
        return PlayMp3Music();

    }

}
class SemiSmartPhone : IRingable
{
    public Sound Ring()
    {
        return PolyPhonicMusic();
    }
}
class BasicPhone : IRingable
{
    public Sound Ring()
    {
        return 8BitMusic();
    }
}

class Main
{
    public static void Main(string[] args)
    {
        List<IRingable> phones = new List<IRingable>();
        IRingable htc = new SmartPhone();
        phones.Add(htc);
        IRingable samsung = new SemiSmartPhone();
        phones.Add(samsung);
        IRingable nokia = new BasicPhone();
        phones.Add(nokia);
        // Lets Ring them all
        foreach (IRingable phone in phones)
        {
            phone.Ring();
        }
    }
}

public class MyWrapper : IDisposable
{
    private Session _mySession = new Session(); // This is the COM object

    public void Open() { _MySession.Open(); }
    public void Close() { _MySession.Close(); }

    public void Dispose() {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    public void Dispose(bool disposing) {
        _MySession.Close(); // Calling close without having called open is OK
    }
    ~MyWrapper() { Dispose(false); }    
}

//Username, Host and Password
string path = @"\\192.168.0.5\Share";
string userName = @"User";
string userPassword = "Password";
string userHost = "192.168.0.5";

//The Credentials
NetworkCredential theNetworkCredential = new NetworkCredential(userName, userPassword);
CredentialCache theNetCache = new CredentialCache();
theNetCache.Add(new Uri(path), "Basic", theNetworkCredential);

var g = theNetworkCredential.GetCredential(new Uri(path), "Basic");
var tt = CredentialCache.DefaultNetworkCredentials;

StorageFolder folder = await StorageFolder.GetFolderFromPathAsync(path);
IReadOnlyList<StorageFile> fileList = await folder.GetFilesAsync();
//Try to get all files
foreach (StorageFile sF in fileList)
{
    //do something with sF.DisplayName;
}