Several questions come to mind, Colleen. How many cats are using the box? How often is the box cleaned? What is the box itself made of? What kind of litter are you using? The more cats using the box, the more often it must be cleaned/scooped (at least once/day if multiple cats are using one box) for odor control. If the box is plastic and can be emptied and thoroughly scrubbed occasionally, that's good. Cardboard or something else that will absorb smell - not so good. Having been a petsitter in the past and having had cats for decades, I feel qualified to say kitty litters are NOT all the same. Some are quite good at controlling odor; other litters are pretty much useless at odor control. My favorite is Fresh Step scoopable. It's not very "green", but sometimes we have trade-offs. Cleanliness (meaning scooping often) and a good litter are essential to odor control. Scratch one of your kitties under the chin for me. :-)
Experiment with different brands of cat litter. Some absorb odors better than others. I prefer the Fresh Step brand. Depending on how many cats you have, you may need several litter boxes. I have 1 cat and 2 litter boxes. If your cat(s) will tolerate a covered litter box (mine will but some won't), that will help confine odors. Baking soda can be sprinkled in the litter to help decrease odor. Occasionally dump all the litter, wash out and disinfect the box and add new fresh litter. And this goes without saying...the more cats you have, the more often the litter box(es) will have to be cleaned. If the smell is permeating the house, the litter box is likely overdue for cleaning...unless the cat has urinated on something else (carpet, furniture, a piece of clothing etc.) and you just assume the smell is coming from the litter box.
Edited to add: I didn't see Marilyn's comment until after mine posted but we've said essentially the same thing...right down to preferring the same brand litter. :)
I use Tidy Cat for two cats in a larger box. Clean daily and there shouldn't be a smell. I also toss out the litter fully once a week and clean the plastic box with Fabulouso
We have 2 plastic litter boxes and switch them out regularly. However, we use oil absorbent dry sweep from the auto store mixed with 1/2 cup of baking soda. The dry sweep is very inexpensive and we use baking soda for a variety of household cleaning issues. Have 1 cat that seems to be allergic to almost every commercial made brand of cat litter available, which was getting old, fast. The vet suggested we try the dry sweep, and work like a charm. And the real plus is they are no longer throwing litter out of the box. LOL
We have been using the Arm & Hammer 'quick clump' for one very persnic++
3kity [sp?] kitty. She really like to have it cleaned after every use, but not always possible. This litter seems to control the odor the best too. She likes to 'lead' us to the box to show us what she has done then use it again right after we clean it. occasionally I use an A & H deodorizer that you can sprinkle & mix in for a light scent.
I have one cat, use tidy cat litter, scoop daily, once a month , empty the old litter, wash the litter box with vinegar and hot water, rinse, dry and add new litter. I have no cat smell in the home
i put a layer of old news paper on the bottom with baking soda in between the paper to absorb the odor change weekly scoop daily add a little baking soda
I keep baking soda in a large salt dispenser near the litter boxes...awesome results