What term describes any speech sounds that can be distinguished from one another?

The term that refers to any speech sounds that can be distinguished from one another is "phonemes." Phonemes are the individual sounds of speech that contribute to meaning in a language. For example, the words "bat" and "pat" differ by only one sound at the beginning, which affects their meaning; this difference highlights the significance of phonemes in distinguishing words in a language.

In the context of this question, the other terms serve different functions: diphthongs refer to complex vowel sounds that begin with one vowel sound and glide into another within the same syllable, consonant blends involve combinations of consonants that are pronounced together in a syllable, and onset pertains to the initial consonant sound of a syllable. While all these terms relate to the sounds of language, they do not encompass the broader concept of distinct speech sounds as phonemes do. Thus, phonemes represent the foundational building blocks of spoken language that allow for meaningful communication.