The Collaborative International Dictionary of English v.0.48:

Mellifluous \Mel*lif"lu*ous\, a. [L. mellifluus; mel, mellis,
   honey (akin to Gr. ?, Goth. milip) + fluere to flow. See
   Mildew, Fluent, and cf. Marmalade.]
   Flowing as with honey; smooth; flowing sweetly or smoothly;
   as, a mellifluous voice. -- Mel*lif"lu*ous*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mellifluous
    adj 1: pleasing to the ear; "the dulcet tones of the cello"
           [syn: dulcet, honeyed, mellifluous, mellisonant,
           sweet]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "mellifluous":
   accordant, achingly sweet, affable, agreeable, agreeable-sounding,
   ambrosial, amiable, amicable, appealing, ariose, arioso,
   bittersweet, blissful, candied, canorous, cantabile, catchy,
   cheerful, compatible, complaisant, congenial, cordial, desirable,
   dulcet, en rapport, enjoyable, euphonic, euphonious, euphonous,
   fair, fair and pleasant, felicific, felicitous, fine, fine-toned,
   genial, golden, golden-tongued, golden-voiced, good, goodly,
   gracious, grateful, gratifying, harmonious, heart-warming, honeyed,
   honeysweet, likable, liquid, melic, melliferous, mellifluent,
   mellisonant, mellow, melodic, melodious, music-flowing, music-like,
   musical, nectareous, nectarous, nice, pleasant, pleasant-sounding,
   pleasing, pleasurable, pleasure-giving, pleasureful, rewarding,
   rich, sacchariferous, satisfying, silver-toned, silver-tongued,
   silver-voiced, silvery, singable, songful, songlike, sonorous,
   sour-sweet, sugarcoated, sugared, sugarsweet, sugary, sweet,
   sweet and pungent, sweet and sour, sweet as sugar, sweet-flowing,
   sweet-sounding, sweetened, sweetish, syrupy, tunable, tuneful,
   welcome