Your baby's crying spells have different sounds because they mean different things. Baby might be hungry, thirsty, tired, frustrated, wants to suck on something or needs a diaper change. If you haven't already done so, you will soon learn to distinguish what your baby's cries are trying to tell you. (Read more about learning to differentiate Baby's cries here.)
Another possibility: studies reveal that babies will often cry right before starting a period of developmental growth. Your intuitive little baby senses the change that's coming up and will cry as an expression of the excitement or fear as well as other emotions that accompany the change.
Babies who cry and fuss before bedtime might be trying to discharge the wound-up energy that's been gathering up during the day. It's a way Baby has to settle himself down for a night's sleep. (The good news says that babies who fuss to a greater degree usually sleep more soundly—and longer—overnight).
Babies pick up on their surroundings. They know when you've had a stressful day or when others in the family are feeling blue. This sensitivity will sometimes cause them to cry. It could be Baby just wants to be held. For nine months she was in a warm and peaceful environment known as the comfort of the womb. Now she's sleeping in a crib where people, lights, experiences, and sounds come and go out of her control. Wouldn't you want someone to hold you?