Well you got a lot of advice but I had to read this thread since your title sounded like me a few days ago. It has been humid here, and hot, and I have foster kittens in one room of my house, an adult cat and two kittens of my own.
I am concerned about spraying products into the air, because they are just kittens and one of them is sick. In fact, with three total kittens sick, and smelly poop, well, you know.
I solved my odor problem with baking soda. I am meticulous about cleaning up the recent accidents also.
A good product I recently found is called "Pet Fresh" Odor Eliminator Plus Pet Hair Release.
It looks like a big can of Arm & Hammer Baking soda. You spray it on the carpet and in five minutes vacuum. It really freshened up the place perfectly but baking soda itself may also work just fine. Haven't tried that yet on the carpets. The stuff really does help getting the pet hair off the carpets too.
For an instant fix get a can or two of Tidy Cat deodorant. You put a couple tablespoons into the litter box and it sweetens the whole area. But really, also buying a better quality litter is key. I was trying to cut corners and buy cheap litter since I was going through it so fast with all the kittens, but in the long run I am saving money buy using the expensive litter.
I would LOVE to be able to flush litter but don't trust my own septic system. So, what I do is use a gawd awful amount of grocery bags and keep them in a bin next to each litter box (one for each cat plus one). I keep a separate scooper near each box too, to make life easier. After cleaning the litter boxes five plus times a day I take the bag outdoors and put it in a SEALED container, like a five gallon bucket until trash day then I put it out. This is to keep the flies away from my house and it works well. I don't tolerate flies well at all and had a recent maggot problem when I kept the trash inside the house in a sealed container. Just don't do it--get the poo out of the house.
I'm guessing you have more than one indoor cat. I never had a smell when it was just me and Pixel. Now it's a little battle. The better food must help but right now I can't tell. I got a sick kitten pooping all day and night. Good luck.