Laudator Temporis Acti

Vergil, Aeneid 6.264-267 (tr. H. Rushton Fairclough):

Ye gods, who hold the domain of spirits! ye voiceless shades!
Thou, Chaos, and thou, Phlegethon, ye broad, silent tracts of night!
Suffer me to tell what I have heard; suffer me of your grace
to unfold secrets buried in the depths and darkness of the earth!

di, quibus imperium est animarum, umbraeque silentes,
et Chaos, et Phlegethon, loca nocte tacentia late,        265
sit mihi fas audita loqui; sit numine vestro
pandere res alta terra et caligine mersas!

Nicholas Horsfall on umbraeque silentes:

Cf. 432f. silentum / consilium, Matius fr. 8 at maneat specii simulacrum in morte silentum with Courtney's n. (probably gen. plur.; he renders 'an image of the appearance of those silent in death'). Cf. Licinia Ricottilli, EV 5*, 12, Norden, p. 209, Bömer on Ov. F. 2.609 (with further bibl.), Setaioli, EV 2, 956, Cumont, LP, 70 and notably J.N. Bremmer, Early Greek concept..., 84f.. Unsurprisingly, No[rden] turns to 'silence' in magical texts (and indeed comms., apart from Page, not helpful on this point), but some understanding of the spirits' 'silence' emerges from more familiar authors:

(i) It has of course been noted that in many texts the souls are not silent at all, in that they make a good deal of noise (but Serv. on 264 nam hominum umbrae loquuntur seems anomalous) but that noise is far from human speech and clearly reflects theriomorphic views of the soul. Notably as bee; see n. on 707-9 and, more fully, Vergilius 56 (2010), 39-45 at 40f.; whence the souls buzzing at Soph. fr. 879 Radt. But also birds, Rohde, Psyche, 2, 371, n.2, Cumont, LP, 293-302, Dodds, Greeks and the irrational, 162, n. 38, comms. on Od. 11.605f., Plin. Nat. 7.174 (Aristeas), n. on 309-12. So the cheeping and twittering indicated by Hom. τετριγυῖα, Il. 23.101 (with Diog. Laert. 8.21). Not to mention bats, Od. 24.5, 9, Tert. An. 32.3, ad fin., M. Wellmann, PW 6.2741.38ff., Bettini (707-9), 225f. and n. on 283 for Lucian, VH 2.33. Cf. too the extraordinary cry of Od. 11.43. In one sense, therefore, 'silence' suggests inability to communicate in human speech; cf. the phantasm of Aen. at 10.639f. (Juno) dat inania uerba, / dat sine mente sonum.

(ii) Here, comms. naturally compare 432 silentum (that is, roughly, 'the dead'; OLD s.v., §2) and 492f. pars tollere uocem / exiguam (strongly suggestive of Hom., just cited); cf. too Hes. Scut. 131 (death that steals the voice), Theogn. 568f. (when dead, as a voiceless stone). Cic. TD 1.37, CLE 1552.38. On the other hand. complete silence would be inimical to the plot, whether in Od. 11 (cf. Page, Hom. Odyssey, 24) or here; so in Hom., blood endows the dead with speech (cf. A. Heubeck on Od. 10.516-40), while in V. the dead speak as required, and Elysium seems particularly vocal. The absence of laughter (Bremmer, Early Greek concept..., 85f.) not directly relevant, but it does seem to fit in easily with the ancient apophatic vein (cf. (426-547) and vd. Page here; see too Johnson, 88-90) in characterisation of the Underworld: absence of strength (Od. 10.521, etc.), of colour (e.g. 272, 480 (where vd. n.), G. 1.277), of substance (269, 292, 413), of light, (265, 267, 270, G. 4.472), of touch (700-2), and naturally of sound (see too infra, tacentia). No essential inconsistency, but a variety of ways of conceiving the Underworld.

Related post: Hellish Noise.