WordNet sense 1 (an unchanging intonation):

drone, droning

WordNet sense 2 (of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value):

monotonic

WordNet sense 3 (sounded or spoken in a tone unvarying in pitch):

flat, monotonous

WordNet sense 4 (characterized by lack of variation in pitch, tone, or volume):

unmodulated