Check if it's muted in the control panel. Check Device Manager for any devices with errors. Remove and re-install any that are. When did this start? Anything happen before it stopped working?
In the Dell Webcam Manager, click the Dell Webcam Center.
In the lower right-hand corner of the Dell Webcam Center window, click the drop-down arrow next to Audio Source and click Digital Mic (Realtek High Definition Audio) to enable the microphone.