The Collaborative International Dictionary of English v.0.48:

Euphonious \Eu*pho"ni*ous\, a.
   Pleasing or sweet in sound; euphonic; smooth-sounding.
   --Hallam. -- Eu*pho"ni*ous*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

euphonious
    adj 1: having a pleasant sound; "a euphonious trill of silver
           laughter" [syn: euphonious, euphonous] [ant:
           cacophonic, cacophonous]
    2: (of speech or dialect) pleasing in sound; not harsh or
       strident; "her euphonious Southern speech"

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "euphonious":
   achingly sweet, agreeable, agreeable-sounding, appealing, ariose,
   arioso, balanced, canorous, cantabile, catchy, concinnate,
   concinnous, dulcet, euphonic, euphonical, euphonous, fine-toned,
   flowing, fluent, golden, golden-tongued, golden-voiced, harmonious,
   honeyed, measured, melic, mellifluent, mellifluous, mellisonant,
   mellow, melodic, melodious, music-flowing, music-like, musical,
   ordered, orderly, pleasant, pleasant-sounding, rich, silver-toned,
   silver-tongued, silver-voiced, silvery, singable, smooth,
   smooth-sounding, songful, songlike, sonorous, sweet, sweet-flowing,
   sweet-sounding, symmetrical, tripping, tunable, tuneful