IPhone 5s Volume fades in and out while watching video

When watching a video on social media, youtube, or online the sound starts fine, then fades out for a second, then back to normal. It can be very annoying, especially watching short clips. This is a new phone from Apple, and don't recall having this issue with any of my past IPhones.

I don't use headphones, only speaker.

Any suggestions?

thanks.