Audio problem: When the Mexicans begin their final attack, we see Travis call to Joe to wake him up, but if you watch closely, you don't see his lips move, you just hear "Joe."
Audio problem: The messenger at the beginning leaps off his horse and says, "They're all dead. Alamo." Watch closely and you'll see his lips don't match "Alamo" and he keeps speaking without making a sound. (00:02:25)
Answer: Yes, the battle of San Jacinto really did last 18 minutes. Houston's army surprised the Mexican army while they were taking an afternoon nap.