There are many things that can cause our voices to be hoarse (rough-sounding), but it is impossible to say for sure why your baby is hoarse without knowing more about her and examining her.
The voice box is made up of the vocal cords and the nerve that makes them move, which together allow us to speak and make sounds. Anything that causes the voice box not to work the way it usually does, affecting either the vocal cords or the nerve, can change our voices and make us sound hoarse. Some of the more common things that can cause hoarseness include infections and injury.
Infections that involve the throat — for example, the common cold — also affect the area around the voice box. In addition to nasal congestion and fever, these infections can cause swelling and inflammation of the vocal cords, causing hoarseness or even sometimes temporary loss of the voice (laryngitis). This hoarseness should improve as the infection clears.
With any injury or trauma to the voice box (for example, if your baby needed to have a breathing tube inserted to help her breathe), the vocal cords could be damaged. In addition, any stretching of the neck during the birthing process also could stretch the nerve supplying the voice box. Both of these things will cause hoarseness, but fortunately both get better with time. Finally, if the thyroid gland (in the front of the neck) was very large or something else in the neck (for example, a large lymph node or tumor) presses on the area of the vocal cords, hoarseness can result.
If your baby has been hoarse since birth and she is otherwise healthy (growing well, no cough, no difficulty breathing), then it is fine to watch her over a period of time until you mention it at her next checkup. If this is not the case, then I think it is important for you to take your baby to the pediatrician to have her checked sooner.