Nikon D3100 problems?

Okay, so I have a Nikon D3100 camera, and I usually use it to record videos. I haven't recorded a new video for about a month or so, and today I recorded a video, but when I would play it back, there would be no sound. I only noticed this now (because like I said, I haven't recorded in a while), but when I play the video on my computer, there would be sounds. Can anyone help me? How do I get the sounds to playback on my camera? It's not on mute by the way.

Do the older videos playback on your camera with sound, or just the ones you recorded recently?

If none of them have sound, you have either inadvertently turned the function off, or the camera is malfunctioning.

If so, you can try resetting the camera and see if that corrects the problem.