Lucan’s unforgettable description of Erictho’s final addition to her witch’s brew at (6.~685). I’d like to remember to check a commentary to see what predecessors and parallels he could have been working with – Hesiod’s Typhoeus (passage below) is the only other extensive description of a voice that comes to mind but there’s the key difference that in Hesiod the sounds are present only one at a time (the list presenting him another lovely little ἄλλοτε alliterative catalogue) whereas Lucan bundles them into some unimaginable end. The closest I get is to a nightmare version of polyphonic overtone singing
and lastly [she mixed in] her voice, more powerful than any drug to bewitch the powers of Lethe, first uttered indistinct sounds, sounds untunable and far different from human speech. The dog’s bark and the wolfs howl were in that voice; it resembled the complaint of the restless owl and the night-flying screech–owl, the shrieking and roaring of wild beasts, the serpent’s hiss, the beat of waves dashing against rocks, the sound of forests, and the thunder that issues from a rift in the cloud: in that one voice all these things were heard.
Tum vox Lethaeos cunctis pollentior herbis
Excantare deos confundit murmura primum
Dissona et humanae multum discordia linguae.
Latratus habet illa canum gemitusque luporum,
Quod trepidus bubo, quod strix nocturna queruntur,
Quod strident ululantque ferae, quod sibilat anguis;
Exprimit et planctus inlisae cautibus undae
Silvarumque sonum fractaeque tonitrua nubis:
Tot rerum vox una fuit.
Hesiod at Theogony ~830:
φωναὶ δ᾽ ἐν πάσῃσιν ἔσαν δεινῇς κεφαλῇσι
παντοίην ὄπ᾽ ἰεῖσαι ἀθέσφατον: ἄλλοτε μὲν γὰρ
φθέγγονθ᾽ ὥστε θεοῖσι συνιέμεν, ἄλλοτε δ᾽ αὖτε
ταύρου ἐριβρύχεω, μένος ἀσχέτου, ὄσσαν ἀγαύρου,
ἄλλοτε δ᾽ αὖτε λέοντος ἀναιδέα θυμὸν ἔχοντος,
ἄλλοτε δ᾽ αὖ σκυλάκεσσιν ἐοικότα, θαύματ᾽ ἀκοῦσαι,
ἄλλοτε δ᾽ αὖ ῥοίζεσχ᾽, ὑπὸ δ᾽ ἤχεεν οὔρεα μακρά.
And there were voices in all his dreadful heads which uttered every kind of sound unspeakable; for at one time they made sounds such that the gods understood, but at another, the noise of a bull bellowing aloud in proud ungovernable fury; and at another, the sound of a lion, relentless of heart; and at anothers, sounds like whelps, wonderful to hear; and again, at another, he would hiss, so that the high mountains re-echoed.