Why do all languages sound different?

I’m going to answer a different interpretation of this question. If all languages have access to the same, finite repertoire of segments (phonemes), then why do they sound as different as they do?

There are several answers to this.

• The repertoire of phonemes may be finite, but the realisation can be phonetically different. A Dutch /x/ is much more fortis than a Greek /x/.
• Different languages employ quite different subsets of the available phonemic inventory.
• Languages differ in sound, not only at the level of individual segments, but also in how they arrange those segments, their phonotactics. People are very attuned to phonotactic differences, because that’s what they are listening for when they are trying to make sense of strings of segments as words.
• Languages, dialects, and for that matter idiolects differ hugely in their suprasegmental phenomena, the aspects of speech that range beyond the individual segments. That includes intonation, loudness, and timbre.