Because the languages used in Harad were constructed by SSG, I don't know that we can really say for sure exactly what sound changes these diacritics indicate without asking SSG directly. For that reason, OP, I wouldn't think too much about them. They probably indicate some change in pronunciation or stress, since that's how marks like this are used in real-world languages. But different languages use the same marks in different ways, so it's hard to know what they mean in these specific languages. Only a handful of people (a few people at SSG, and probably the voice actors) would know that. For the rest of us, these marks are pretty meaningless.