I think making characters “sound”, for lack of a better word, different when they speak is a lot harder in genre fiction than it is for other forms of fiction. If you’re writing a story in the real world in the modern day, the reader can more easily pick up on how a character sounds based on region, ethnicity, language background and so on.
When you’re writing genre fiction, all of that goes out the window. The reader, at first, has no clue about this world. The various nuances of speech in this world become unfamiliar.
So how do we accomplish making character sound different without beating a dead horse?