The Collaborative International Dictionary of English v.0.48:

Quaver \Qua"ver\, v. i. [imp. & p. p. Quavered; p. pr. & vb.
   n. Quavering.] [OE. quaven to shake, to tremble; cf. LG.
   quabbeln to shake, to be soft, of fat substances, quabbe a
   fat lump of flesh, a dewlap, D. kwabbe, and E. quiver, v.]
   1. To tremble; to vibrate; to shake. --Sir I. Newton.
      [1913 Webster]

   2. Especially, to shake the voice; to utter or form sound
      with rapid or tremulous vibrations, as in singing; also,
      to trill on a musical instrument
      [1913 Webster]

WordNet (r) 3.0 (2006):

quavering
    adj 1: (of the voice) quivering as from weakness or fear; "the
           old lady's quavering voice"; "spoke timidly in a
           tremulous voice" [syn: quavering, tremulous]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "quavering":
   ague, aspen, blurred, breathy, bumpiness, chattering, choked,
   choking, chorea, cold shivers, croaking, didder, disquiet,
   disquietude, dithers, drawling, drawly, dysphonic, fidgetiness,
   fidgets, fits and starts, flutter, guttural, harsh, hawking,
   heaving, hoarse, inarticulate, indistinct, inquietude, jactation,
   jactitation, jerkiness, joltiness, lisping, mispronounced, muzzy,
   nasal, palpitation, palsied, palsy, panting, pitapat,
   pitter-patter, quaking, quaver, quavery, quiver, quivering,
   quivery, restlessness, shakes, shaking, shaky, shiver, shivering,
   shivers, shivery, shudder, shuddering, snuffling, spasms, stifled,
   strangled, succussatory, succussion, succussive, thick, throaty,
   throb, throbbing, tremble, trembling, trembly, tremor, tremulous,
   tremulousness, trepidation, trepidity, twangy, twitter, unrest,
   velar, vibrating, vibration, wobbly