Alice: Hey, boys. Is that any way to treat a lady?
Chris Redfield: Behind those doors is a UPV. Urban Pacification Vehicle. Prison kept it in case of a serious riot. 16 wheeler, steel plated, water cannon, seats 20. 10 tons of fun. Roll on out in style, drive right over them.
Luther West: We're going to need some more firepower.
Chris Redfield: I got you covered. When I was stationed here, we used a room in the basement as an armory. It's got every weapon you can dream of.
Angel Ortiz: How do you know your unit didn't take 'em with them when they pulled out?
Chris Redfield: Because by the end, there were a lot more guns than there were people to use them.
Albert Wesker: Well isn't this one big family reunion. Chris and Claire Redfield... You've really become quite an inconvenience for me.
Alice: I told you I'd be bringing a few friends.
Albert Wesker: You should've brought more.
Alice: Day 6,1800 hours, Los Angeles. No signs of life, not even the undead. They must have burned with the city. But what about the rest?
Albert Wesker: I'm what you used to be... Only better.
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.