noun (n)

• the quality of wearisome constancy, routine, and lack of variety(noun.attribute)

  Example:
  He had never grown accustomed to the monotony of his work.
  

  Synonym:
  humdrum, sameness
  

  source: wordnet30
• constancy of tone or pitch or inflection(noun.attribute)

  source: wordnet30
• A frequent recurrence of the same tone or sound, producing a dull uniformity; absence of variety, as in speaking or singing.(noun)

  source: webster1913