The Collaborative International Dictionary of English v.0.48:

Tuneful \Tune"ful\ (-f[.u]l), a.
   Harmonious; melodious; musical; as, tuneful notes. " Tuneful
   birds." --Milton. -- Tune"ful*ly, adv. -- Tune"ful*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tuneful
    adj 1: having a musical sound; especially a pleasing tune [syn:
           tuneful, melodious] [ant: tuneless, unmelodious,
           untuneful]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "tuneful":
   achingly sweet, agreeable, agreeable-sounding, appealing, ariose,
   arioso, canorous, cantabile, catchy, dulcet, euphonic, euphonious,
   euphonous, fine-toned, golden, golden-tongued, golden-voiced,
   harmonic, honeyed, melic, mellifluent, mellifluous, mellisonant,
   mellow, melodic, melodious, music-flowing, music-like, musical,
   pleasant, pleasant-sounding, rhythmic, rich, silver-toned,
   silver-tongued, silver-voiced, silvery, singable, smooth, songful,
   songlike, sonorous, sweet, sweet-flowing, sweet-sounding, tunable,
   tuned