Nintendo reconfirmed during their corporate strategy meeting that Splatoon 2 will feature voice chat. The official Splatoon JP twitter account went on to confirm this information, as well as other details.
They tweeted that:
The Splatoon twitter stated that voice chat will be functioned via the mobile application as previously confirmed. The app and the game will work together. You will be able to talk everyone before the match starts, once the match starts you can only communicate with your team. You cannot talk with complete strangers.
So it looks like voice chat will only work for people you know unlike other consoles which allow free voice communication between strangers.