Princeton's WordNet

1. monotone, drone, droning(noun)

   an unchanging intonation

   Synonyms:
   bourdon, trailer, poke, droning, dawdler, monotone, pilotless aircraft, drone, drone pipe, lagger, laggard, radio-controlled aircraft

   Antonyms:
   modulated

2. monotone(adj)

   a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)

   Synonyms:
   drone, droning

   Antonyms:
   modulated

3. monotonic, monotone(adj)

   of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value

   Synonyms:
   flat, monotone, monotonous, monotonic

   Antonyms:
   modulated

4. flat, monotone, monotonic, monotonous(adj)

   sounded or spoken in a tone unvarying in pitch

   Synonyms:
   plane, savourless, flavorless, categoric, matt, humdrum, insipid, level, monotonic, compressed, flat, unconditional, matted, vapid, categorical, monotonous, two-dimensional, matte, monotone, savorless, prostrate, bland, mat, flavourless

   Antonyms:
   modulated