Voice chat is a setting available for training tables and team matches.

At a teaching or bidding table, there is a Voice button visible on the table by default. The table host can use it to add themselves as a speaker and allow / disallow other speakers.

When creating a team match, you can choose to allow Voice Chat in your team match settings, in the Options tab:

A few clarifications about how Voice works, in the games where it is enabled

• Voice chat only works per table, not at both team match tables. A TD cannot send voice chat to all players, they have to join a table, and add themselves as a speaker to that table in order to chat.
• When there are active speakers at your table, the Voice button turns yellow. When there are no players speaking, the Voice button is blue.
• Voice chat is not active by default. Each player must click to activate voice first. If they do not click the Voice button, they will not hear the other speakers -- even if the others are already chatting.
  

How to start speaking or listening to the other players

Click the Voice button, which is located at the bottom right corner of the Chat area:

At the top left corner of the screen, you will see a pop-up listing the other players -- if any of them have also clicked Voice -- with a "Start speaking" button:

In team matches, or at training tables where the host has added you as a speaker, you will see options to Start speaking. Otherwise, you have options to hear the other speakers.

A yellow message will appear, informing you that you may speak, if you wish:

Click Start speaking to join the voice chat. This will allow you to speak and also to hear the other speakers.