As a commercial (indie, but I make money) game developer who mostly uses CC/PD art and a little royalty-free paid work:

All commercial games MUST credit the artists and musicans. Paid or not. It's fine if that's buried in the credits screen that nobody looks at, but it had better be there. I've seen companies destroyed over failing to attribute. You can't afford the lawyer time to deal with it.

Glad to see your stuff up here, Eric, I'll be using some of these!