The buzzing sound is heard when the bee makes the air vibrate in some way. It used to be thought that the movement of the wings caused the sound. However the bees can buzz even when the wings are at rest. So it is probably the vibration of the muscles in the thorax causing the thorax to vibrate that makes the buzzing sound.
Actually it can do both, but it mandibles (jaws) are not strong enough to cause any pain to a human, and are used mainly for moulding the wax for the cocoons, honeypots and pollen stores. Also the mandibles are rounded, so have next to no piercing ability. Some species, mainly Bombus lucorum, do use their mandibles for nectar robbing, but it takes them ages to pierce through a flower to steal the nectar, so human skin is in no danger. This is also why a bumblebee nest in the house will cause no damage as they cannot drill or dig. Workers (females) and queens can sting (see this page for photographs etc.), and they have an unbarbed sting , so could sting repeatedly,unlike honey bees, but rarely do. The only time I have heard of bumblebees stinging is when they have been roughly handled, or their nest or the entrance to the nest is being tampered with. I have never been stung, and I have handled hundreds, measuring tongue lengths, head widths etc. I can honestly say they are the most accommodating insects to work with - hence my deep fondness for them.
No. Bumblebees, like all other insects, have what is called an exoskeleton. This simply means that the skeleton is on the outside of the body, unlike our skeleton which is on the inside. The skeleton of the main part of the body is made up of hard plates of chitin held together by more flexible thinner sections of chitin. This allows for some expansion and movement, but not much real growth. This is why, during the larval stage, the insect has to moult to grow bigger. And once the insect has emerged as an adult that is as big as it gets.