Some videos i have taken has no audio.

I went to a concert and took A LOT of videos. i opened camera, selected “photo” instead of video, changed exposure to -2.0, dragged the capture button to the right to keep the setting while on video. i did that for all of the videos ive taken BUT !! there are 5 videos, taken in between the show, that does not have an audio. i tried to unmute, turn off bluetooth, turn off and on the ringer button, power off my phone, but it still has no audio. the videos i took after these 5 videos were perfectly fine. can anyone help me? my ios is up to date too.