The Collaborative International Dictionary of English v.0.48:

Unmelodious \Unmelodious\
   See melodious.

WordNet (r) 3.0 (2006):

unmelodious
    adj 1: not having a musical sound or pleasing tune [syn:
           tuneless, untuneful, unmelodious] [ant:
           melodious, tuneful]
    2: lacking melody [syn: unmelodious, unmelodic, unmusical]
       [ant: melodic, melodious, musical]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "unmelodious":
   absonant, atonal, cacophonous, cracked, diaphonic, disconsonant,
   discordant, disharmonic, disharmonious, dissonant, flat, grating,
   harsh, immelodious, inharmonic, inharmonious, musicless,
   nonmelodious, off, off-key, off-tone, out of pitch, out of tone,
   out of tune, raucous, sharp, shrill, sour, strident, tuneless,
   unharmonious, unmusical, untunable, untuned, untuneful