An AU file is an audio file type only used with Audacity. Audacity is a free, cross-platform audio editing software. AU files are created automatically when you either create a new project or import an audio file into Audacity. Creating a new project generates a myproject_data folder, which stores that project’s AU files. How Do You Open an AU File? AU files are not opened individually. They are part of Audacity projects. These are stored as AUP for older Audacity files, or AUP3 if created with version 3.0.