After the events of Guardians of the Galaxy, fans wondered whether Groot would either stay a baby or grow at such an accelerated rate that he'd be Vin Diesel sized again. Alas, we got our answer, and Groot is still a wee sapling in Guardians of the Galaxy Vol. 2, which of course raises the question of how he'll sound vocally. This is a question that only James Gunn could answer, and he's certainly having fun withholding the answer.
Our Sean O'Connell ran into Gunn at San Diego Comic Con, and posed the question to the director as he was celebrating a reel showcasing his return to the Marvel Cinematic Universe. But, seeing as the decision of whether Groot will have a baby or manly voice isn't an easy one to crack, James Gunn had the following to say:
We were real up in the air about whether we were going to put it in [the footage], but we'll leave that for a different day. We're working it out!
So while it sounds like James Gunn and company wanted to show off the possibly new, or possibly old, voice of Groot in Guardians Of The Galaxy Vol. 2, the decision to hold off on such an unveiling sounds like it's a well-kept secret for the film's ultimate release. Of course, considering how secretive Marvel Studios can be when it comes to their future films, there's a chance that the decision wasn't Gunn's to make, and he's making light of the scenario. However, listening to James Gunn deliver the above quote, which you can experience in Sean's interview below, it looks like the voice is set, and it's just a matter of deciding when the public is ready to hear it.
Obviously, the decision involving which voice Baby Groot will use in Guardians of the Galaxy Vol. 2 is something that's important to the character's return to the screen. Should the character act more like a baby incarnation of itself, with juvenile behavior that matches its smaller stature, a more child-like voice of a higher pitch would be a better fit. But if Groot will still be kicking ass and using his smaller size as a tactical advantage in the name of defeating the villains, then Vin Diesel's booming vocals will be the way to go. Either way, the voice is as important as the decision to keep Groot in his baby faced form.
Ultimately, looking at the scenario from the outside, both options have their drawbacks and advantages. But to have a baby Groot with baby vocals is probably not the best way to go about things, as the stature of the character shouldn't be oversold to the audience. Whichever option lands its way into the final film, Guardians of the Galaxy Vol. 2 will revel in said decisions on its May 5, 2017 release date.