But these differences wouldn't matter to our understanding of the word that they each said. We still hear 'Hello'. In other words, there are some differences that are not meaningful differences in the sense that they don't make a new word in English. This is because the sounds you are using, despite the little differences in stress, intonation, pitch, volume, speed, and so on, are all meaningful in some way. Here we are in the subject area of phonology.
Phonology is the study and identification of the meaningful sounds of a language.
A sound is meaningful within a particular language if, when it is changed, the word also changes. Changing 'chin' into 'gin' alters its meaning. In Czech, however, there are several different ways of making ch-like sounds which do make a difference to the word. Conversely, in English we have a meaningful distinction between the 'l' sound and the 'r' sound, whereas Chinese and Japanese do not. It's time now to do some practical work on sounds.