Have you noticed how your voice gets lower when you have a cold?
You may already know that our voices originate from the vibration of the vocal cords in the larynx, or "Adam's Apple." The pitch range of our voices is determined by the length and thickness of our vocal cords. Shorter and thinner vocal cords vibrate faster, and result in a higher voice like a child's or a woman's. Longer, thicker vocal cords vibrate more slowly and make a lower voice like a man's. A boy's voice gets lower at puberty because his vocal cords get longer and thicker.
Here is what happens when you get a cold. With a cold comes inflammation of the structures around the vocal cords. With the inflammation comes swelling of those structures. As stated previously, thicker vocal cords vibrate more slowly and give a lower pitch. Our inflamed, swollen--that is thicker--vocal cords vibrate more slowly than normal, and our voice is lower than it usually is.