If you want just your employees to be able to download it, just run a website on your company server if you have one (see xampp if you don't already have one running on at least one computer in your office), put the song file on the server and post a link on the server to the song file. Then they can connect to the server and download the song (and no one not connected to the company network can).
Welcome to Android Central! And all they need to do is move that ringtone file to the /Ringtones directory using any file manager app. It will then show up as an option when they go to the ringtone selection menu.