You may find that not all movies in the Italian iTunes Store have original audio. Some do, some don’t. That is the decision of the regional distributor/studio representation. The languages available with each movie are always listed on the movie’s Details page.
For many Hollywood movies, you will find that Italian dub may be default, but English audio is also available.
For steaming movies, you can select a different language from any of the included language tracks. Change subtitles or languages for videos in the Apple TV app or iTunes Store - Apple Support
For file downloads (in contrast with online streaming), a subset of the language support is included, as set in the app’s preferences/settings. If original audio is not available with a particular movie for your region, then it will likewise not be included in the download. Try adding a specific language to the settings, if it is known to be available but somehow not retrieved as ‘original’ (e.g. if primary/default is instead misinterpreted as original.)
Please list the movie for more specifics.