Question

Why the sound of baby is feeble

Answer 1

The amplitude of vibration is less.

The loudness of sound depends on the amplitude of the sound producing object's vibration. If the amplitude of vibration is more the sound will be louder. It is feeble for small amplitude.

If the amplitude of vibration is doubled the loudness increases 4 times. If the amplitude of vibration is halved the loudness becomes 1/4th.

The unit used to express loudness of sound is decibel or Db. When the loudness of sound is above 80 Db it becomes physically painful. When it is at the level of 140 Db sound hurts too much. 

Answer 2

The amplitude of vibration is directly proportional to sound. As the amplitude increases, loudness also increases. And when the amplitude is less, the sound produced is feeble. This phenomenon is applied in case of a baby.
The frequency of vibration of vocal cords increases with the increase of pitch of shrillness of voice, as we move from man to woman and then a small baby.
Hope this will prove helpful for you.