Hello and welcome to the Forums, Niceone
The general belief that giraffes are mute is a myth... they are not. Their voices may not be too impressive, but they do make sounds.
We had a nice discussion about this on the forums about a year ago but unfortunately the thread is no longer there.
I did however save part of my posts which I am quoting below:
Harrison, D.F. (1980). Biomechanics of the giraffe larynx and trachea, pp 258-264:
Despite possession of a well developed larynx and a gregarious nature, the Giraffe is able to utter only low moans or bleats. Morphological and histological examination, together with measurements of trachea and subglottic area, on three fresh larynges (Giraffa camelopardalis) has made it possible to explain the lack of vocal power. Factors such as thoracic expiratory flow rate, length of trachea and recurrent laryngeal nerves, together with morphological details of vocal folds, and intrinsic laryngeal muscles have all been considered, providing a unique example of the relationship between morphology and function in the mammalial larynx.
Jolly, L. (2003). Giraffe Husbandry Manual, p 11:
(link no longer available)*)
Giraffe are silent, but not mute. Despite possessing a well developed larynx, the giraffe is only able to utter low moans and grunts. Morphological examination of the larynx and trachea indicate factors such as the thoracic expiratory flow rate and the length of trachea and laryngeal nerves are the reason for the lack of vocal power in the giraffe (Harrison 1980). The larynx is smaller than that of a horse, vocal folds and laryngeal ventricles are absent.
National Wildlife Magazine, Dec/Jan 2004, vol. 42 no. 1:
Liz von Muggenthaler, a bioacoustical researcher, has graphic proof that giraffes may actually "talk" to each other. "We believe that giraffes are forcing large columns of air out their long, long trachea and out a small opening, which is actually their larynx," she told me. It’s not a sound we humans can hear unaided, since it’s at a range beneath our own hearing called infrasound. But von Muggenthaler says that as the air passes through the larynx, it might sound "like a great burst of air: PSSH."
*) Did anyone by chance save the Giraffe Husbandry Manual (PDF-file) to their HD at the time? Mikev, Craig, Imberbe? I'm afraid I didn't...