Test microphone with bundled Sound Recorder. If microphone works well, please check if there is problem with the receiver of recipient's phone.
Enable amplicfiation function during a call on recipient's phone to check if he can hear your voice.
Reboot your phone.
Update firmware to latest version: Go to Settings > About > System update > Check Update.
Access Safe Mode by long pressing power key > tapping and holding Power Off > tapping OK. If your phone works well in Safe Mode, there may be some 3rd apps that cause malfunction. Please reboot your phone and check again if it works well.