IPA: /ˈtoʊn.ləs.li/
KK: /ˈtoʊnˌlɛsli/
In a way that lacks tone or emotion; without variation in pitch or expression.
She spoke tonelessly, making it hard to tell if she was happy or sad.
Tonelessly → It is formed from "tone" (from Old English *tōn*, meaning sound or musical pitch) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "tonelessly" means in a manner that is without tone or sound.
Think of being 'without' ('-less') a 'sound' ('tone') — that's why tonelessly means doing something without any sound.