If you have enabled pre-orders on iTunes, by default, customers will be able to listen to 30 second previews during the pre-order period on all Apple platforms that you have delivered the release to:

If you wish to disable the ability for customers to listen to previews before the official release date, this can be set via the 'Pricing' tool. Under the 'iTunes Pre-Orders' section, set 'Enable Previews' to False: