Please have your demonstration as an audio file. You may also link to demonstrations on other file hosting sites, provided that the file should stay on the site indefinitely. I don’t want someone clicking on a demo only to find a 404 error.
If you put up a demo, put it in a folder with the game’s name, like “dissidia Final Fantasy/Dissidia.mp3”. This is so that if others make more demos, they can simply be put into the folder and linked to from the games-list page.