Most likely, the damage is mostly superficial.
What you should look for is swollen wood, soft wood, and for the thing physically coming apart. Check the bottoms in particular, as they could have extra damage if they were on thick wet carpet for an extended period of time. In other words, you are looking for the same things you would look for with water damage.
The more imbedded the urine, the more damage there will be, and the more difficult it will be to repair, as well as the more difficult it will be do get rid of the smell.
Obviously, you would want to inspect the speakers in all the normal ways as well, for "foam rot" (if applicable) and functioning of the speakers themselves.
I would also try to talk them down on the price, though they may be worth the asking price if there are no serious problems and you are willing to deal with refinishing them and taking care of the smell.