answer:Two guesses (having learned a little mandarin, but not being a scholar of the language): 1. Context: I’d imagine it’s similar to when listening to punk rock or heavy metal, it takes a surprisingly little bit of word to be able to figure out what words mean by context. 2. Tones are a lot more subtle when spoken by a fluent speaker than when explained to or by a westerner. It’s probably (likely) possible that they can be worked into the sung lyrics without affecting the sound too much. Like rolling your r’s or enunciating your t’s while singing…