The Collaborative International Dictionary of English v.0.48:

Vox angelica \Vox` an*gel"i*ca\ [L. angelica angelic.] (Music)
   An organ stop of delicate stringlike quality, having for each
   finger key a pair of pipes, of which one is tuned slightly
   sharp to give a wavy effect to their joint tone.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

vox angelica
    n 1: an organ stop producing a gentle tremolo effect [syn: vox
         angelica, voix celeste]