Adjective
Having no tone; unmusical.
Source: Webster's dictionaryAnd the great spirit of darkness spread a shroud over me... everything was silent-everything. But upon the heights soughed the everlasting song, the voice of the air, the distant, toneless humming which is never silent. Knut Hamsun
his toneless mechanical voice Source: Internet
Likewise, Chinese reporters abroad may file their stories in toneless pinyin. Source: Internet
Syllables can sometimes be toneless and are described as having a neutral tone, typically indicated by omitting tone markings. Source: Internet
Thus, the Proto-Athabascan word *tuː ('water') is toneless toː in Hupa, high-tone tó in Navajo, and low-tone tù in Slavey; while Proto-Athabascan *-ɢʊtʼ ('knee') is toneless -ɢotʼ in Hupa, low-tone -ɡòd in Navajo, and high-tone -ɡóʔ in Slavey. Source: Internet
Tone may simply be ignored, as is possible even for highly tonal languages: for example, the Chinese navy has successfully used toneless pinyin in government telegraph communications for decades. Source: Internet