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]