I am not a vet, but I think I can tell you this from my mother's tubing experience. After having surgery that requires intubation, the throat always gets torn up. My mother sounded like a woman who had been smoking for 100 years after a surgery from the breathing tube. It took like 2 weeks I think for her to get her voice back all the way.
Morphine slows everything down too. Anesthesia and pain killers slow the intestines to the point some people need stool softners after surgery. Combine this effect with the scratched up esophogus and I think it would be a reasonable guess to say the throat is sluggish after being tubed and having the morphine too.
I would make sure your kitten doesn't have fluid in the lungs though. Did your vet listen to his/her chest? If not, maybe you should ask about it and if the office doesn't resspond to you- I would talk to another vet's office for a second opinion.
In fact, you may want to call another vet's office anyways and just ask if your kittens symptoms seem normal, as you are wanting to confirm what your current vet is telling you.
Good luck