Can't hear recorded sound during video playback

I take a lot of videos of my kids. I have noticed lately that the volume is low or muffled...you can't hear what is being said even if they are screaming 6 inches away. I thought maybe something had gotten into the microphone, etc. I recorded a voice memo and on playback i could hear what was recorded just fine at a normal level. Now once I tried to record a video of my kids talking, it played back so low that i can't hear anything and that is with the volume turned wide open. Not really sure what this issue could be but if the volume works fine on voice memo wouldn't it work the same on video playback?