Symbolism, Imagery, Allegory
Let those bells ring, Shmoopsters, because Princess Augusta is back. After his daughter's (purported) death, King Augustine ordered that the bells in the tower of the Old Juliana palace to never ring again since they were his girl's favorite. The silencing of the bells, then, symbolizes the way the king literally shut down when he believed his daughter to be dead—while the bells remain silent, the king is incapacitated… and Cabbarus is in charge.
Once Mickle/Princess Augusta returns, though, the bells begin to ring again. Theo initiates their ringing, albeit unintentionally, when he chases Cabbarus up to the bell tower. But no matter—it's a sign of a new era, a return to the good old days when King Augustine ruled and his daughter was alive and well. The bells are alive with the sound of music:
The guards were racing up the steps into the belfry, with Musket scuttling ahead of them. The dwarf's mouth was open, shouting. Theo heard none of his words. Above him, the bell had stirred into life, its voice resounding in the others that hung beside it. The clangor exploded in his ears. He was being hoisted up and pulled back over the railing, still gripping Cabbarus. Theo's hand had frozen on the man's arm. Someone was prying his fingers loose. (25.27)
Once the bells ring, it's a sign that life will be restored to King Augustine through his daughter's resurrection. And, insofar as their silence symbolizes Cabbarus's manipulative power grab, when they ring out again, their sounding represents the end of Cabbarus's terrible rein.