SS will not pick up code that is in a folder which has no _config.php file, hence putting it into mysite
so in short, move your HomePage.php to the mysite/code folder
In the folder miisound/, did you create an empty _config.php file? The presence of a _config.php file tells SilverStripe that there is stuff in the code directory to process. If you don't have the _config.php file, SilverStripe will simply ignore anything in the code directory.