It's very easy, but you will need to download the file from iTunes first.
Follow these steps:
In iTunes click "Get" on the file you want transcribed
Once the file has been "Gotten", click on 3 dots to the right of that file and click "Download"
Once the file is downloaded, click "Locate in Finder"
Once you see it in your Finder you can simply drag it onto your upload page in Sonix: https://my.sonix.ai/upload