Sound detection settings

I had set up my camera to detect sound and notify long ago. I’ve always receive the notification for sound just as I received the notification for motion but now when I get the notification for sound and I play the video, I hear nothing at all like not a peep but if there is motion and I play the video I hear the sound just as well as see the video. Why does my sound no longer play when I get the notification and play the video. It’s silence like mute.