Machine learning really doesn't have many uses for games outside of the development cycle (or non-critical gimmicks). If it's exposed to player input - or especially; random input - it absolutely must have robust, stable and controllable behaviour, at least on paper. Most of the time getting any of those out of an NN is either difficult or blatantly impossible.

Though speech synthesis specifically might be a good use case for some simpler ML, such as to form the final sound from processed phonemes (i.e acting as a fuzzy lookup table).