Even though giraffes are very quiet animals, they are quite capable of vocalizing. However, it's rare for the adults to vocalize. Here's footage of a baby vocalizing during a vet exam (begins at 1:44). Watch the full video if you want to hear more about the myth that giraffes are mute:
Here's another sort of noise that giraffes make. When competing for females, males will participate in a form of combat known as "necking." They will shove each other and hit each other with their "horns"--known as ossicones, they are not like true horns and are unique in the animal world. The ossicones are bony projections of the skull covered in a layer of skin. It's actually pretty brutal, with each hit producing a very loud thud. A competitor can be injured, knocked unconcious, or even killed.