The Collaborative International Dictionary of English v.0.48:

Sonorous \So*no"rous\, a. [L. sonorus, fr. sonor, -oris, a
   sound, akin to sonus a sound. See Sound.]
   1. Giving sound when struck; resonant; as, sonorous metals.
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   2. Loud-sounding; giving a clear or loud sound; as, a
      sonorous voice.
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   3. Yielding sound; characterized by sound; vocal; sonant; as,
      the vowels are sonorous.
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   4. Impressive in sound; high-sounding.
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            The Italian opera, amidst all the meanness and
            familiarty of the thoughts, has something beautiful
            and sonorous in the expression.       --Addison.
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            There is nothing of the artificial Johnsonian
            balance in his style. It is as often marked by a
            pregnant brevity as by a sonorous amplitude. --E.
                                                  Everett.
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   5. (Med.) Sonant; vibrant; hence, of sounds produced in a
      cavity, deep-toned; as, sonorous rhonchi.
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   Sonorous figures (Physics), figures formed by the
      vibrations of a substance capable of emitting a musical
      tone, as when the bow of a violin is drawn along the edge
      of a piece of glass or metal on which sand is strewed, and
      the sand arranges itself in figures according to the
      musical tone. Called also acoustic figures.

   Sonorous tumor (Med.), a tumor which emits a clear,
      resonant sound on percussion.
      [1913 Webster] -- So*no"rous*ly, adv. --
      So*no"rous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sonorous
    adj 1: full and loud and deep; "heavy sounds"; "a herald chosen
           for his sonorous voice" [syn: heavy, sonorous]

Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "sonorous":
   Gongoresque, Johnsonian, achingly sweet, affected, agreeable,
   agreeable-sounding, appealing, ariose, arioso, aureate, bedizened,
   big-sounding, bombastic, booming, canorous, cantabile, catchy,
   clangorous, clattery, consonant, convoluted, deafening,
   declamatory, droning, dulcet, ear-piercing, ear-rending,
   ear-splitting, earthshaking, elevated, euphonic, euphonious,
   euphonous, euphuistic, fine-toned, flamboyant, flaming, flashy,
   flaunting, flowery, forte, fortissimo, full, fulsome, garish,
   gaudy, golden, golden-tongued, golden-voiced, grandiloquent,
   grandiose, grandisonant, high-flowing, high-flown, high-flying,
   high-sounding, highfalutin, honeyed, inkhorn, labyrinthine,
   lexiphanic, lofty, loud, loud-sounding, loudish, lurid,
   magniloquent, melic, mellifluent, mellifluous, mellisonant, mellow,
   melodic, melodious, meretricious, monotone, monotonic,
   music-flowing, music-like, musical, noiseful, oratorical, orotund,
   ostentatious, overblown, overdone, overelaborate, overinvolved,
   overwrought, pealing, pedantic, piercing, plangent, pleasant,
   pleasant-sounding, pompous, pretentious, pulsing, rackety,
   resonant, resonating, resounding, rhetorical, rich, ringing,
   rolling, rotund, round, sensational, sensationalistic, sententious,
   showy, silver-toned, silver-tongued, silver-voiced, silvery,
   singable, songful, songlike, soniferous, sounded, sounding,
   stentoraphonic, stentorian, stentorious, stilted, sweet,
   sweet-flowing, sweet-sounding, tall, throbbing, thunderous, tonal,
   toneless, tonitruant, tonitruous, tortuous, tunable, tuneful,
   uproarious, vibrant, vibrating, window-rattling