Roblox on Xbox has an integrated voice chat system that allows you to chat with your friends and party members.
If a microphone is connected via a headset or Kinect, you are automatically placed into voice chat with other players in the game you joined.
Note: Voice communication must be enabled in Xbox Settings
Xbox Live Party Chat
Roblox also allows for the use of Xbox Live Party Chat with your Xbox Live friends. When active, this feature automatically overrides in-game chat.
Note: You can switch to in-game chat by going to the "Your party" panel in Xbox and selecting "Switch to game chat".
For more information about Party Chat on Xbox along with troubleshooting steps, please see the following articles: