The Collaborative International Dictionary of English v.0.48:

Vibrancy \Vi"bran*cy\, n.
   The state of being vibrant; resonance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vibrancy
    n 1: having the character of a loud deep sound; the quality of
         being resonant [syn: plangency, resonance,
         reverberance, ringing, sonorousness, sonority,
         vibrancy]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "vibrancy":
   bassness, deepness, fluctuation, frequency, frequency band,
   frequency spectrum, fullness, harmonic motion, hollowness,
   libration, lowness, mellowness, nutation, oscillation, pendulation,
   periodicity, plangency, resonance, resonance frequency, richness,
   sonority, sonorousness, vacillation, vibratility, vibration,
   wavering