The sound system of a language is referred to as:

The sound system of a language is known as phonology. Phonology studies how sounds function within a particular language or languages, encompassing the rules that govern the sound patterns, phonemes, and their organization. This includes understanding how sounds interact, how they can change in different contexts, and how they are perceived by speakers of that language.

In contrast, syntax pertains to the rules that govern sentence structure and the arrangement of words to create meaning. Semantics is the study of meaning in language; it examines how words, phrases, and sentences convey meaning. Morphology focuses on the structure of words and the formation of word forms, including prefixes, suffixes, and the way these components interact. These distinctions highlight why phonology is specifically the term that addresses the sound system of a language.