Actually it seems like the entire speech is a fabrication. There are two texts attributed to Sphrantzes, but the longer one with the speech is considered to be based on another source and not actually by Sphrantzes. As for what Sphrantzes did write, all he said on this was (PDF available here):

On Tuesday May 29 [1453], early in the day, the sultan took possession of our City; in this time of capture my late master and emperor, Lord Constantine, was killed. I was not at his side at that hour but had been inspecting another part of the City, according to his orders. Alas for me; I did not know what times Providence had in store for me!

So where did the speech come from? It seems to be copied from a letter by Bishop Leonardo Giustiniani, who was present in the city during its fall. I can't ascertain whether his account is accurate or not, but there is good reason to doubt it. To quote Marios Philippides, who wrote the most recent scholarly book on this topic, The Siege and the Fall of Constantinople in 1453 (2011): :

Was there in fact a last celebration attended by the emperor in Hagia Sophia? Did the emperor actually address his Greek and Italian barons in the palace before the general assault? Was there really an opportunity for dramatic speeches? Aside from Leonardo, who has a flair for the emotional, other eyewitnesses fail to mention such moving scenes. And there is every reason to conclude that Leonardo has provided his own free embellishment of the facts. [...]

The plain fact is that Constantine had abandoned his imperial quarters at the palace of Blakhernai, which had been turned over to the Venetian bailo and his troops during the siege. And we do know from other eyewitness sources that Constantine XI had erected a tent to house himself and to serve as his headquarters in the enclosure between the great and outer walls at this late stage in the drama. The emperor and his commanders, who had been continually repairing the collapsed defenses with their troops and workers, would have had no opportunity to assemble for last-minute processions, speeches, and farewell scenes, however moving and dignified they might be. In all likelihood, they were all too busy supervising the last-minute repairs that must have been going on at a feverish pace, as the general assault of the Turks was expected. If any speeches were made, they would have been very short and hastily improvised at the critical sector. If any church services were conducted for the troops and commanders of the land sectors, they took place in the vicinity of the walls and not in Hagia Sophia at the tip of the Golden Horn. We can only conclude that Leonardo paints this fictional scene in the ancient cathedral and in the imperial palace in order to add nobility, atmosphere, and pathos to his narrative, for he wished to wrap the slain emperor in a shroud of tragic dignity.