Her sensations were indefinable, and so were they a few minutes afterwards upon hearing Henry Crawford, who had a chair between herself and Tom, ask the latter in an under voice whether there were any plan for resuming the play after the present happy interruption […]1814 May 9, [Jane Austen], chapter II, in Mansfield Park: […], volume II, London: […] [George Sidney] for T[homas] Egerton, […], →OCLC, page 40:
Brayder introduced them to one or two of the men, hastily and in rather an undervoice, as a thing to get over.1859, George Meredith, chapter XXXVI, in The Ordeal of Richard Feverel. A History of Father and Son. […], volume (please specify |volume=I to III), London: Chapman and Hall, →OCLC:
3 more examples
A thousand soft undervoices that jumped my jangled senses from his last, weakly syllabled wind to a mosaic of voices within voices, each one immanent in the other, none his but all strangely his […]1990, Charles R. Johnson, Middle Passage, Simon & Schuster, published 2012, page 171:
Through the battered century of world wars and massive violence by other means, there had always been an undervoice that spoke through the cannon fire and ack-ack and that sometimes grew strong enough to merge with the battle sounds. It was the struggle between the state and secret groups of insurgents, state-born, wild-eyed—the anarchists, terrorists, assassins and revolutionaries who tried to bring about apocalyptic change.1997, Don DeLillo, part 5, chapter 4, in Underworld, part 5, New York: Scribner
[…] Your mother Played Prospero, flying her magic in To stage the Masque, and bless the marriage, Eavesdropping on the undervoices Of the honeymooners in Paris […]1998, Ted Hughes, “Setebos”, in Birthday Letters, Faber & Faber:
undervoice
verb
/ˌʌndə(ɹ)ˈvɔɪs/
Etymology From under- + voice.
1
To voice too weakly.
Consequently, the danger for Dutch learners of English is undervoicing the English lenis fricatives rather than the reverse.1984, Beverley Collins, Inger Margrethe Mees, The Sounds of English and Dutch, page 51:
2
To make a quieter or background sound beneath.
Undervoicing the flame, there was the popping of hollow weed stalks, the tinkle of woody stems crisping and falling in coals.1902, Martha McCulloch-Williams, Next to the Ground: Chronicles of a Countryside, page 74: