Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

   NOUN:FAINTNESS &c. adj.; faint sound, whisper, breath; undertone, underbreath; murmur, hum, buzz, purr, lap [of waves], plash; sough, moan, rustle, susurration [rare], tinkle; “still small voice.”
  HOARSENESS &c. adj.
  SILENCER, muffler; soft pedal, damper, mute, sordino [It.], sordine [all music].
   VERB:WHISPER, breathe; mutter (speak imperfectly) [See Stammering]; susurrate [rare].
  MURMUR, purl, hum, gurgle, ripple, babble, flow; rustle, tinkle.
  steal on the ear, melt in the air, float on the air.
  MUFFLE, deaden, mute, subdue.
   ADJECTIVE:FAINT, scarcely -, barely -, just- audible; low, dull; stifled, muffled; inaudible; hoarse, husky; gentle, soft; floating; purling, flowing &c. v.; muttered; whispered &c. v.; liquid; soothing; dulcet (melodious) [See Melody. Concord]; susurrant [rare], susurrous [rare].
   ADVERB:IN A WHISPER, with bated breath, sotto voce [It.], between the teeth, aside; piano, pianissimo [both music], sordamente [It.], sordo [It.], à la sourdine [F.]; out of earshot; inaudibly &c. adj.
  1. The mingled notes came softened from below.—Goldsmith
  2. The beetle winds His small but sullen horn.—Collins
  3. A little noiseless noise among the leaves, Born of the very sigh that silence heaves.—Keats
  4. ’Twas whispered in heaven, ’twas muttered in hell, And echo caught faintly the sound as it fell.—Fanshawe