Yeah, i'll try the free music archive, great tip for finding music for youtube videos!
And yes, the sign asks the pet owners to collect their pet's excrements, as it is an area intended to be a playground for dogs.
It's a great spot for flying microquads. I go there only at night because there are no pets and it's well lit (otherwise i can barely fly in the dark) This is a big city and it's not easy to find spots for flying. Law is not very favorable to drones in Spain, even when it comes to 30gr micros. Not as crazy as it is in Canada right now, but its very restrictive, and you're not supposed to be flying anything into a populated area, so... night is my friend