I've also been annoyed by this. I am fairly certain it's to do with the fact that the videos in Stories play automatically, and videos that play automatically are muted/silent as a matter of policy in Safari (which I generally find to be a good thing!).
Unfortunately, Instagram doesn't provide any video controls in the Stories interface, so there's no way to interact with the video to turn the sound on.
There is a workaround, but it's a bit fiddly: You need to setup a bookmarklet that triggers fullscreen for the currently playing video via Javascript, and then you can use the Apple-native controls in the video player to turn the volume up. But you have to do this for every single story. Like I said: it's a bit fiddly.
If you haven't setup a javascript bookmarklet before this post on Reddit has the steps:
https://www.reddit.com/r/ipad/comments/87cog9
The javascript in my bookmarklet is slightly different though. Here for reference:
JavaScript:
javascript:document.querySelector(%22video%22).webkitEnterFullScreen();
Hope that helps!
Cheers