Audio problem: Near the beginning of the film when Alice and her clones break into the Umbrella building, she comes down in an elevator. The numbers above the elevator showing what floor she is on beep every time they change, but some of the beeps are out of sync. A beep is heard for floor 6 before the number 7 has even changed and a beep is also heard a fraction of a second before the number 3 fully appears. (00:07:05)
Audio problem: As Alice, Claire, Chris, Luther and Kim are on the rooftop shooting the zombies as they rush towards them, you can see Chris' lips move as if he is shouting something to the others but no words can be heard coming out. (00:59:10)
Answer: Based on the sounds to go with the scene, I'd say it is zombies taking the guards out. However, considering following events it is more likely that Alice and her friends have something to do with it.