Restore deleted item to Favorites drop-down menu

It depends. If it's one you created, then you have to re-create it yourself. If it's an Adobe one, then you have to go to your preferences folder and delete Favorites.xml - Audition will re-create this next time it starts. There's a sting in the tail, of course - if you created any of your own, then you'll destroy them in the process of re-instating the originals. I dare say that if you're feeling clever, you might be able to edit the original .xml file and extract yours before you delete it, and add them to a new one, but I think you'd need the right editor, and it would be quite a bit of faffing about to do.