The Collaborative International Dictionary of English v.0.48:

Soundness \Sound"ness\, n.
   The quality or state of being sound; as, the soundness of
   timber, of fruit, of the teeth, etc.; the soundness of
   reasoning or argument; soundness of faith.
   [1913 Webster]

   Syn: Firmness; strength; solidity; healthiness; truth;
        rectitude.
        [1913 Webster]

The Free On-line Dictionary of Computing (17 December 2009):

soundness

   The quality of being sound (2).


Moby Thesaurus II by Grady Ward, 1.0:

196 Moby Thesaurus words for "soundness":
   admissibility, advantageousness, agreeableness, aplomb,
   auspiciousness, authenticity, authoritativeness, authority,
   balance, balanced personality, beneficialness, benevolence,
   benignity, body, calculability, canonicalness, canonicity, class,
   cogency, common sense, completeness, concreteness, constancy,
   contact with reality, cool, cool head, coolheadedness, coolness,
   credit standing, density, dependability, desert, due sense of,
   durability, entireness, equilibrium, eupepsia, excellence,
   expedience, fairness, faithworthiness, fastness, favorableness,
   fine fettle, fine shape, fine whack, fineness, firmness,
   first-rateness, force, fullness, glowing health, good condition,
   good constitution, good digestion, good health, good sense,
   good shape, good trim, goodliness, goodness, gospel truth, grace,
   haleness, hardness, healthfulness, healthiness, healthy body,
   healthy constitution, healthy mind, helpfulness, high feather,
   homeostasis, horse sense, imperturbability, intactness, integrity,
   invariability, invincibility, justifiability, justness, kindness,
   level head, level-headedness, levelheadedness, logic, logicality,
   logicalness, lucid interval, lucidity, marbles, mass, materiality,
   mental balance, mental equilibrium, mental health, mental hygiene,
   mental poise, merit, mind, mint condition, nerve, niceness,
   normalcy, normality, normalness, orthodoxicalness, orthodoxism,
   orthodoxness, orthodoxy, palpability, picture of health,
   plain sense, plausibility, pleasantness, plenitude, ponderability,
   practical mind, practical wisdom, practicality, predictability,
   profitableness, quality, rationality, reason, reasonability,
   reasonableness, reliability, religious truth, rewardingness,
   right belief, right mind, rightness, robust health, rootedness,
   rude health, rugged health, sanemindedness, saneness, sang-froid,
   sanity, secureness, security, sense, senses, sensibleness,
   skillfulness, sober senses, sober-mindedness, soberness, sobriety,
   solidity, solvency, sound mind, sound sense, soundness of mind,
   stability, stable equilibrium, stable state, stableness,
   staunchness, steadfastness, steadiness, steady nerves,
   steady state, stoutness, strength, sturdiness, substance,
   substantiality, substantialness, superiority, sweet reason,
   tangibility, temper, the truth, top shape, toughness,
   traditionalism, trustworthiness, undeflectability, unerringness,
   uniformity, unindebtedness, unshakable nerves, unshakableness,
   usefulness, validity, value, virtue, virtuousness, weight,
   wholeness, wholesomeness, wit, worth