Mixup's main feature is it's LUFS aligner that allows you align all versions of the same song all the same level. Additionally, it allows you to align your music to popular streaming levels such as Spotify, Youtube, iTunes etc...
Sadly, due to current limitations on Safari (Desktop and Mobile), this feature is momentarily disabled. In a nutshell, for some reason, Apple still has not successfully implemented the Web Audio API in its browser when all other browsers have done so. The Web Audio API is what allows us to adjust levels and some other cool things in order to maximize the user experience.
Apple is fully aware of the problem and if we're lucky, this will all be fixed before year-end.
Thanks for your understanding.