The Collaborative International Dictionary of English v.0.48:

Toneless \Tone"less\, a.
   Having no tone; unmusical.
   [1913 Webster] Tong

WordNet (r) 3.0 (2006):

toneless
    adj 1: lacking in tone or expression; "his toneless mechanical
           voice" [ant: toned]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "toneless":
   achromatic, achromic, anemic, ashen, ashy, bled white, bloodless,
   cadaverous, chloranemic, colorless, dead, deadly pale,
   deathly pale, dim, dimmed, dingy, discolored, droning, dull,
   etiolated, exsanguinated, exsanguine, exsanguineous, faded, faint,
   fallow, flat, ghastly, gray, haggard, hueless, hypochromic,
   lackluster, leaden, livid, lurid, lusterless, mat, mealy, monotone,
   monotonic, muddy, neutral, pale, pale as death, pale-faced, pallid,
   pasty, sallow, sickly, soniferous, sonorous, sounded, sounding,
   tallow-faced, tonal, uncolored, wan, washed-out, waxen, weak,
   whey-faced, white