We have a cat, named Max, that frequently urinates on the carpet. I use the cat version of Nature's Miracle, and also Rocco & Roxie. They are enzymatic cleaners that break down the components in the urine that smell. Cat urine smell is worse than Dog urine smell.
[5-minute interlude of one of our non-carpet-peeing cats (Frankie) walking in, climbing on my lap, and kneading my stomach. Coincidence? I have to see if she's secretly on Photrio.]
The cleaners don't always completely remove the smell from the carpet, but do a respectable job. They clean much better on "hard" surfaces.
You really have to saturate the area, and let it soak for at least 15 minutes - preferably more. For film you'd want to soak, then wash, then use a PhotoFlo-like final rinse. You may have to do it several times.
However, I don't know if the cleaners will damage the negatives, so it may be a risk, as BrianShaw stated.
On the other hand, you can always store them apart from other negatives, so if you can't get rid of the smell, but CAN get rid of the residue, then using a standard cleaner meant for film would probably be fine, starting with distilled-water soaks to get the easy stuff off. You want to preserve them and possibly print/scan. It doesn't matter if you have residual odor as long as you can keep them from degrading further (which is difficult to know).
Whatever you try, a proper negative wash (or several) should follow, along with re-stabilization if they are color.