Which video is it? Maybe there is no sound for that video?
You say that it is not on mute - do you mean both the video and your device/computer?
That's about all I got for troubleshooting... I've had troubles with certain channels - there was one that I could watch about two minutes of her videos and then the screen would cut to black, but I still had audio (all of her videos had no talking so that wasn't very helpful...) this was the only channel I had a problem with, so I asked her and she said it's an issue on my end... but I know (as a YTer myself) that there are certain settings you put your videos to for best viewing for everyone, and other settings that while are great quality, don't work well for all users. She wasn't willing to give up her quality, so I gave up following her. (Honestly, her videos had a lot of bad info in them anyways, so it wasn't really a loss.)