When downloading an MP4 video, the audio does not work.

I downloaded a Dropbox video with an MP4 format to my camera roll, but when I play it in my camera roll, it shows the video, but not the audio. When I check the volume button to see if it’s muted or not, the button is grayed out completely. I’ve tried downloading the dropbox video to other platforms, and it has the same issue. This does not happen when I do the same to other videos downloaded, and that may be because the video has a larger file size considering that it’s a music video. How do I fix it to where the audio plays?