From Longman Dictionary of Contemporary Englishtonelesstone‧less /ˈtəʊnləs $ ˈtoʊn-/ adjective SLa toneless voice does not express any feelings SYN expressionless ‘I’m sorry, ’ he said, in a flat toneless voice.
Examples from the Corpus
toneless• The chairman's voice was firm but toneless.• His toneless voice caught fire and was blessed with new colour and depth. 