Have you ever been awakened by the phone in the middle of the night? "He-l-l-o?" And you wonder where that froglike voice comes from? Your vocal cords are simply not warmed up yet.
The same thing can happen at the company-site interview. You have little opportunity to actually speak until you arrive at the company-site. And then you are expected to talk nearly nonstop for the remainder of the day.
Take the time to warm up your voice on the way to the interview. If you are driving, turn on a radio station you enjoy and sing along. Top of your lungs is just fine. If you are taking a cab, either spend time talking with the cabbie (they have some of the most interesting stories you will ever hear) or ask to have the radio turned on. Again, sing along—although perhaps a little more quietly than if you were in your own car.
In any case, use and stretch your vocal cords before beginning your day of interviewing. You will benefit with a clear and resonant voice.