Here's why:
* Subject-Verb Agreement: The subject of the sentence is "the sound of bells," which is singular. The verb "was" agrees with the singular subject.
* Collective Noun: "The sound of bells" is a collective noun, referring to a single, unified sound. Even though it's made up of many individual bell sounds, it's treated as one entity.
Therefore, "was" is the correct verb form in this sentence.