“Microsoft Flight Simulator X” attempts to bring the full experience of flying an aircraft to the PC. Much like a pilot in an actual aircraft, you can utilize a headset or microphone to communicate with others who are playing with you during a multiplayer session. If your chat window is closed, your microphone does not work automatically; you’ll need to utilize keyboard commands to use your microphone during a game.

How to Activate a Microphone in  Microsoft Flight Simulator X  - 36