Ents, being tree-people (as opposed to magically animated trees) have either evolved from another life form (a tree or an animal mimicking a tree very well) or were created (either magically or via gods, etc.) dependent on your world's lore.
- If it's a tree-mimic, than standard mammalian vocal cords or the avian equivalent would not be unexpected.
- Tree-based evolution however would be different as they would likely not need to evolve vocal systems outside of standard tree-speak.
- Direct creation on the other hand would likely depend on lore justification. If they're like Tolkien's tree-shepherds, it makes some amount of sense for them to be able to communicate with the other races (especially as the vanilla trees could speak to some extent as well). If it makes sense that they should be able to speak, given their purpose of creation, then let them be heard (whether by vocal cords or wind-voices or tree sign language). Or maybe they were created by a particularly absent-minded god and are completely mute.
Point of interest though, wooden hollows and thin pieces of wood are particularly good at amplifying and resonating sound. So if an Ent had vocal cords, their speech might sound something like a guitar, violin, etc. trying to speak.