You're reading this article in English. If you read it out loud, you'd probably adjust the way you say certain words, like using a rising tone if there was a question or emphasizing different syllables of "complex" depending on whether you were talking about a group of buildings or something really complicated. But even if you didn't — even if you heard it from a robot voice — you'd still understand the meaning.
That's not the case in tonal languages like Mandarin Chinese, Thai, Vietnamese, Hmong, and Punjabi. In those languages, a change in tone comes with a change in meaning. Which begs the question: If singing involves setting words to musical tones, how do you sing in a language that relies on tones for meaning?