Quote: "ask them for permission to use it in a commercial product"
Yes, true, but while making your game it is an even better idea to make sure there is a clear license for the media before you even decide to use it. Otherwise, you could end up having to replace your media after the game is already made. Big headache for any product.
Most of the free media you get from here only require a mention in the game credits, but as Rampage said, always double check.
Many publisher's will also want to know what kind of a fan base you have already established, if you have a website dedicated to the game, what kind of potential your game has for other product marketing, what updates and future plans do you have for your game.
I am sure there are many more things to consider, but these are some of the basics that would apply to looking for distribuion of any product.
