I want to add a short background sound on the website thats starts when loading the site.
All the widgets I found with autoplay possibility don't work on iPad en Iphone.
I doubt you will find a way to do it - not sure you would want to do it either. Quickest way to lose viewers is to surprise them with an unexpected sound.
You can find all of the reliable and tested Muse Widgets on the public market on the Muse Widgets website. You may be able to find something there that will help.