If the microphone difficulty you're having is caused by a post-update issue, it'll most likely be fixed by another update that includes the necessary remedy patch. If this is a widespread issue as a result of the most recent upgrade to iOS 11, Apple should move quickly by releasing a fresh firmware update to fix the problem. This is why it is critical to install any new updates that are available for your device.

1. Go to Settings-> General->Software Update to check for updates. If a new iOS version is available for your iPhone, you'll receive an update notification. However, before you install, make a copy of your present iPhone data for backup purposes.
2. Return to the update menu and press the option to Download and Install the update file on your device after you're finished.