Hey,
Sorry to hear you are having some issues with the mic in-game.
There are 2 suggestions below you can try to resolve this:
1) Please make sure the microphone is set as “Default Communications Device” in the audio settings of Windows to control your communication device.
2) Go to C:\Users\CurrentUser\Documents\My Games\Rainbow Six - Siege\UplayID
Go to GameSettings.ini
Go to Audio
Change "VoiceChatCaptureMode=" to a 1 on the variable.