Screaming requires oxygen.
Fire depletes oxygen, an incinerator requires a jet style oxygen injection to feed it fuel due to its speed of depleting it.
No one being burned alive in an incinerator can scream, they cant get oxygen to do it.
I am going to assume you are not working from experience here. If so, I defer to the guy with more time burning living things.
That said, screaming requires a lung filled with a gas. It doesnt require oxygen. Charles tells us gasses tend to expand greatly when heated. (See Boyles and ask any diver about similar phenom).
So, they may well be able to scream longer than you or I can, until they cant.
