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Primordial as spoken by its native elemental creatures should be rumblings, hisses, crackles, roars, booms, thumps, and scrapes, representing the movement of the elements (earthquakes, lightning, wind, falling pebbles, glacial advance, waterfalls, waves), with a massive variety in volume (depending on context) and taking a *long* time to convey a complete sentence, with massive amounts of meaning in the small details and dozens of different ways to speak the same words.

The humanoid incarnation of the speech would be very difficult and outright *hurt* to speak, due to the unnatural grating sounds that would need to be produced by the vocal chords and the necessary varieties in volume. A speaker would probably focus on a dialect associated with a favored element when speaking, but would have to be able to understand a large variety of dialects to adequately communicate with different elemental creatures.

I would see some anthropomorphized elemental creatures, like efreets, as speaking a more structured dialect of the language which is closer to a humanoid interpretation: more "voice" and less "sounds", faster, more eloquent, more intellectual, more emotional. Archons would also have a faster dialect, but with a perpetual angry and warlike sound about their speech. Titans would speak a form of primordial designed to invoke sheer awe, as appropriate for their intimidating stature and connection to the creation of the world, filled with boasting pride, seeing themselves as god-like.

Abyssal and Giant would both sound somewhat similar, but very distinct.

Abyssal would be warped by sheer unadulterated evil, twisting in upon itself, filled with sounds of violence and anguish. It would have shrieks, howls, deep groans and other genuinely nasty sounds woven through its words.

Giant would be like a severe dumbing-down of Primordial, losing almost all of its elemental sounds and replacing long depictions of formative forces and qualities with short worldly colloquialisms.

I think most of it is lower than humans can hear but i think it would sound like the boom of thunder, the wind and the crackle of fire

The problem with describing Primordial (or any language in D&D, really) as something no human could actually speak is that humans obviously can speak it perfectly well. If you're going to have it as random elemental noises or something similar that isn't reproducible by the human larynx, then you probably should ban humans and other non-native speakers from being able to speak it (they could still learn to understand it, of course).