I'm all for mechanical vocalizations aka "animal noises". Basically, sounds that their physiology allows them to make. And not just via the mouth but any organ that is capable of producing sounds - bugs for example sing by rubbing or flapping their wings at hyperspeeds, a fact that at least Vibrava's/Flygon's 'dex entries seem to take note of.
As for "talking", as in producing human language sounds from mouth, I'd rather much they don't. Pok?mon not being able to trivially communicate with humans is one of the factors that I feel sets apart Pok?mon as individuals from the creatures from other franchises like Digimon and Monster Rancher when treated as individuals.
And finally, telepathic speech, I would presume many if not all Pok?mon are sapient and thus can formulate thoughts; whether enough of them can actually project those thoughts is a matter I usually leave to the level of "importance" of the Pok?mon, from which I thus also assume that it's not merely a projection as "human speech" but more like "universal language" or "transparency of thought".
But for the games? Mechanical vocalizations. It's long past the age of MP3, I don't see why don't we have that already besides "pikause" franchise mascot (like, really).