Hi - can someone help?

I'm have a new computer with Windows 8.1. I'm running Windows Movie Maker Version 2012 (Build 16.4.3522.0110).

I can activate my USB-connected webcam without a problem, but all I have is the video - no audio. The inbuilt mic is working fine - I can record narration without difficulty - and Skype is also OK (both audio and video).

Is there a conflict somewhere that's blocking the audio on a straight recording from the webcam?

Many thanks for any advice, which will be gratefully received!