Sorry, I am not quoting exact suttas, but what comes to mind is that chanting can have some tone of ups and downs to help in the rhythm. However, it shouldn’t go to the level of singing.
Different people have different ways of interpreting that. Some people regard another monk’s chanting as akin to singing, whereas that very monk thinks of other’s chanting as singing.
As Pali spoken language have difference between long and short a, i, u, it’s good to be mindful of the exact long or short sound when chanting it.
Anyway, since the Buddha allowed people to learn the Dhamma in their own language, chanting (in pali) is largely just for tradition’s sake instead of liberation’s sake. It’s possible to chant in English all the way.
Thais have their own standardized chanting manner, Myanmar another way of pronouncing it, Sri Lanka style is the closest to proper Pali pronunciation.