I had the same problem with my Niece.
She was... like you said terrified of loud noises. Music, TV, screaming... all drove her away.
I was worried when we went to WDW. So... I let her choose the rides. Some had loud noises. I warned her of potential noises, and I let her ride with her hands over her ears. That way she was in control over the volume. The first few unexpected noises startled her. But since she was in front of people she didn't burst out crying as usual. And she got tired of covering her ears afterwhile.
The Parade was difficult. Sitting on the curb it was VERY loud. I even covered my ears.
It was a trade off... either ride and be startled or sit on the sidelines. I tried not to coddle her too much. Now, she is again, fearless.