monotone
[noun]
Synonyms: monotony, humdrum, monotonousness
[adjective]
Synonyms: dull, banausic, blah, dim, dreary, humdrum, monotonous, pedestrian, poky, stodgy

[TahlilGaran] English Synonym Dictionary ▲

mon‧o‧tone /ˈmɒnətəʊn $ ˈmɑːnətoʊn/ noun [singular]
a sound or way of speaking or singing that continues on the same note without getting any louder or softer, and therefore sounds very boring:
He answered all the lawyer’s questions in a dull monotone.

[TahlilGaran] Dictionary of Contemporary English ▲