3D FlipBook - http error

Hi there - I am trying to find where the request is coming over with http and not https. Can you help me figure this out? I am getting this:

jquery.min.js?ver=3.7.0:2 Mixed Content: The page at 'https://www.audiovideosystems.com/symphony/' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://www.audiovideosystems.com/wp-content/plugins/3d-flip-book/assets/css/short-white-book-view.css'. This request has been blocked; the content must be served over HTTPS.

on this page:

https://www.audiovideosystems.com/symphony/

Any help would be greatly appreciated!

Jason