Firstly, it would not be Napoleon who would be making the speech, since he does not possess the skill, flair or charm to deliver an effective and convincing speech. We learn in chapter two that Napoleon was not that great at talking, but because he was wilful and stubborn, he usually got what he wanted.
Napoleon was a large, rather fierce-looking Berkshire boar, the only Berkshire on the farm, not much of a talker, but with a reputation for getting his own way.
We further learn in chapter five that Snowball made much better speeches during debates than Napoleon did and could therefore win the animals over. Napoleon, on the other hand, managed in some way to obtain the support of especially the less intelligent animals, such as the sheep. This suggests that he probably used methods other than language to assert his authority. This soon becomes evident when he sets his dogs (puppies he had secretly raised) on his fellow comrades.
The true propagandist in the tale is Squealer. Squealer possessed the charm and wit to persuade almost every animal and he was used by Napoleon to manipulate, mislead and deceive the other animals into believing whatever he wanted them to.
What we learn in chapter two about Squealer is that:
He was a brilliant talker, and when he was arguing some difficult point he had a way of skipping from side to side and whisking his tail which was somehow very persuasive. The others said of Squealer that he could turn black into white.
It is for this reason that Squealer became Napoleon's mouthpiece.
A speech by Squealer about having sold Boxer to the knackers would probably go something like this:
'Surely, Comrades, you do not believe that we sold Boxer to the knacker out of spite or animosity? No, Comrades, no! Nothing can be further from the truth. We discovered the truth about Boxer whilst he was recuperating in hospital, Comrades. And may I mention that we spared no expense in trying to get him better. No expense, Comrades! We sacrificed what we had to ensure his good health.
It was then that we learnt that Boxer had been plotting behind our backs, Comrades! Yes! During his time in hospital, away from prying eyes, he had been secretly meeting with Snowball, Frederick and Pilkington! Indeed, Comrades! They were going to overthrow the farm! Surely Comrades, you would not want that? Surely you would not want to live in anguish and pain again?
As you know, comrades, our Leader has done his utmost to rid Animal Farm of those who have plotted and attempted to betray our cause. He has taken to extreme measures to ensure our safety. We all know that those who confessed were dealt with in the most severe manner. They betrayed us. And so it was with Napoleon.
We know that many of you, Comrades, have grown to love Boxer. To spare you the grief of bearing witness to his punishment, our eminent Leader decided to kill two birds with one stone: get rid of the enemy and spare our loved ones the pain and suffering of witnessing a friend's pain. It is for these reasons, Comrades, that we sold Boxer to the knacker, and for these reasons alone. Surely, you cannot believe otherwise?
The propaganda, falsehoods and deception are all elements of such a speech. Also, there is the threat of things becoming what they had been. Most of the animals would accept what Squealer says.