Road to Freedom by Gabrielle Davis

Trained as a spy since childhood, Issanka uses her powers over light and sound in service of the higher castes. A caste-less urchin herself, Issanka knows she must be careful never to get too involved in citizen business.

But when the newly crowned King sends an army to tour the country, Issanka no longer has a choice. The journey will take her to a city of wealth and mystery, where the line between castes is no longer clear, and where Issanka must step out of the shadows if she is to
survive.