are you on VPN?
else try the following:

I cannot hear anyone in-game and they cannot hear me back.
If you're experiencing problems connecting to voicechat, try syncing your clock. We timestamp an authentication key locally, so if your clock is out of sync, it can cause the key to be invalid. We're working on a fix for this so it no longer relies upon your local clock, but until then you need to make sure it's not out of sync by following these instructions:
Open Settings.
Click on Time & Language.
Click on Date & time.
Under “Synchronize your clock”, click the Sync now button.

If that does not work set your speaker and mic as default devices and default communication devices.
If that does not work you might want to try to restart your computer and/or set the folder containing FCT as an *exception *for your Firewall/VPN/AntiVirus.