2 Answers

It might not be "urine" If it really really stinks it's probably cat spray from an unneutered male cat. That stuff will glow chartreuse if you hold a black light to it. Male cats spray on the wall not on the floor because of their anatomy. (I wouldn't know how that works with a sofa) There are all sorts of cleaning stuff at the pet store.  If it's pee alcohol or liquid soap might work like bleep suggested.

After cleaning it, you  can rub peanut butter or olive oil on the sofa.

If it was my LEATHER sofa, I'd call the cleaners. 

Itsmee, owner of a male cat who is neutered but manages to spray anyway. 

if its a leather sofa then all you can do is wipe the are with alcohol or with liquid soap a few times until all the smell disappears. urine doesn't get soaked in the leather, it's just dries on it, so it's actually easier to remove the smell from leather than from a carpet for example.