I believe there is a brother and a sister, and they live in one of the poorer communities, and there is a singing statue or something? I distantly remember it from when I was younger... There was a bottom level called the unspeakables or something or other... I really would like to read them again. I think it was a trilogy and one of them had speaker or singer or something in the title, and another had fire in the title?
Sounds like "The Wind Singer" by William Nicholson. It was the first of the Wind on Fire Trilogy, and the sequels were called "Slaves of the Mastery" and "Firesong."
According to Wikipedia, the twins are named Kestrel and Bowman.