The American electronica musician Adam Young of Owl City has revealed on his discography page that he created at least some of the new system sounds in iOS 7. Under "Artist," Young lists "Apple" and under "Year" and "Label," Young lists "2013" and "Sounds for iOS7."
Besides completely remastering virtually all of the previous iOS's system sounds, many new ones were introduced, including a number of ringtones and system alerts. It's not entirely clear if Young created all the new iOS 7 sounds, or just some of them.
Besides his work with Owl City and Apple, Young has also done a number of sound work on animated films including Wreck-It Ralph and The Croods.