My Nikon D3100 doesn't record sound?

Yes the sound button is on and when i film video and play back the audio sign has a line throught which means no sounds, does anyone know how to fix it?

Your camera IS recording sound. You just have the playback speaker turned off/muted.

"film video"? You can't do that. You can record video but film is a completely different media. Where did you hear that? Film video? Please, if you're going to talk digital cameras at least get your terms in order.