Audio file created by Audacity, a free, cross platform audio editor; uses a proprietary format that can only be opened by Audacity; saved as one of multiple audio data files for an Audacity project (.AUD).

When you created a project in Audacity and import audio files, the audio data is converted to the AU format and is stored in a folder with the project called myproject_data, where the project name is myproject.aud. When done editing a project, you can export the audio to several standard formats, including .MP3, .WAV, and .M4A. NOTE: Audacity's AU format is not compatible with the standard AU audio file format.