Peter Harvey in an Introduction to Buddhism says of chanting
[chanting] has a rhythm which encourages the mind to flow from word to
word [but ..] usually lacks melody which might encourage some words to
chanting is neither singing or a monotonous dirge [...] it holds
interest with it's minor variations of pitch and rhythm
So from this I would suggest that the tone of chanting is an aid to memory and interest and doesn't, in English, have a semantic content deriving from the original language. Of course Thai being a tonal language (i am not any kind of expert here) the tones may well have a more semantic content.
I guess less charitably you could say we are making it up a bit. Certainly from my own experience chanting within the Triratna community we have a tone they we vary widely from and add harmonies to on occasion. The the Padmasambhava chant we actually got the words wrong too initially (50 years ago) but we have just stuck to our original bad interpretation. We kind of like it.