Emme Stone's low-pitched voice is actually a result of a health issue when she was a baby!

Emma Stone’s low pitched voice is from being colicky as a baby. Baby colic is when a healthy baby cries on a regular basis for no clear reason. Oftentimes, the colicky baby cries at the same time on a daily basis. Evening time, after meals is an especially common time for babies with colic to cry. 

Normally, babies are only colicky for the first month or so of life, although some babies can have colic for up to a year. Emma had a particularly long colic period.

Her voice was actually very affected by her colic as a baby because it created nodules and left callouses on her voice cords. This caused her to have a low, huskier voice. 

Here are some other fun facts about Emma Stone:

• Emma Stone is generally thought to be a redhead, but she is actually a natural blonde. 
• For her first role as a teenager, she dyed her hair dark brown. For her role in Superbad, she died her hair red. Since then she re-dyed her hair blonde for the Amazing Spiderman. 
• Stone’s real name is actually Emily Jean Stone. The reason that she goes by Emma Stone is because the Screen Actors Guild already had an Emily Stone registered, so she had to pick a new name. 

(Source)