I open the portal and choose to enable voice chat from the options. However, as soon as I enter a duel (whether another player is present or not), if I look in my options menu, I see that the voice chat has been disabled. My preference is not carrying over from the portal into the match. Similarly, if I enable voice chat during a duel, as soon as we move to a rematch, it is disabled again. It's certainly annoying to have to keep re-enabling it, and if it's the same for all players, I imagine that's why voice chat is rarely done.