The term "more sing-song" implies a tone that's lively, musical, or lilting in nature. The opposite of these characteristics would be something that's monotone, flat, or dreary. Therefore, antonyms of "more sing-song" could include words like dull, lifeless, drone, monotonous, or somber. While a sing-song tone can be playful or joyful, an opposite tone can reflect sadness, boredom, or seriousness. However, it's important to note that communication can be nuanced and varies depending on context, intention, and culture. Therefore, using antonyms of "more sing-song" would only apply to specific situations where that style of tone is inappropriate or counterproductive.