AUP (Audacity Project format)

Audacity projects are stored in an AUP file, which is a format that has been highly optimized for Audacity so that it can open and save projects extremely quickly. In order to achieve this speed, Audacity breaks larger audio files into several smaller pieces and stores these pieces in a directory with a similar name as the project. For example, if you name a project "chanson", then Audacity will create a project file called chanson.aup which stores the general information about your project, and it will store your audio in several files inside a directory called chanson_data. The Audacity Project format is not compatible with any other audio programs, so when you are finished working on a project and you want to be able to edit the audio in another program, select Export.

WAV (Windows Wave format)

This is the default uncompressed audio format on Windows, and is supported on almost all computer systems. Audacity can read and write this format.

AIFF (Audio Interchange File Format)

This is the default uncompressed audio format on the Macintosh, and it is supported by most computer systems, but it is not quite as common as the WAV format. Audacity can read and write this format.

AU (Sun Au / NeXT Audio)

This is the default audio format on Sun and NeXT computers, and it is usually very slightly compressed, so it is not a very high quality format. This format was one of the first audio formats supported by Web browsers, and it is still often used for short sound effects where quality is not as important. Audacity can read this format.

IRCAM (Institut de Recherce et Coordination Acoustique/Musique)

This is a very general file format that is commonly used by computer music researchers. Audacity can read this format.

MP3 (MPEG II, layer 3)

This is a compressed audio format that is a very popular way to store music. It can compress audio by a factor of 10:1 with very little degradation in quality. Audacity can both import and export this format. For more information on how to export MP3 files from within Audacity, see Exporting MP3 Files.

Ogg Vorbis

This is a new compressed audio format that was designed to be a free alternative to MP3 files. Ogg Vorbis files are not as common, but they are about the same size as MP3 with very similar quality (and getting better). Audacity can import this format, but it cannot export it.

Sound Designer II

Used only on Macintosh computers and in large part in a Protools environment, which is a professional audio recording and editing solution. This format stores its audio data (PCM only) in the data fork and the format information, such as sample rate, bit depth and mono/stereo choice, in the resource fork. Protools can only work with mono SDII (single channel) files, thus it distinguishes stereo or multichannel files by adding little extensions like .L and .R for stereo files. 5.1 file groups get the suffixes .L(left-front), .C(center), .R(right-front), .SL(surround left), .SR(surround right) and .LFE(low frequency effects).
Audacity won't actually write this format, but you can read files in this format via the Import Raw Data command in the Project menu.

WMA (Windows Media Audio)

This is a proprietary compressed audio format created by Microsoft. Unlike Ogg Vorbis, which is a completely free open standard, or MP3, which is protected by patents but is still an open standard, the Windows Media Audio format is completely proprietary. Because Microsoft does not share its file format information, Audacity cannot open WMA files, nor can it save them. Unfortunately, many WMA files can never be converted to another format, because the format was designed to be copy-protected. Microsoft and the Recording Industry don't want you to be able to take WMA files and burn them onto CD's, so they make it virtually impossible to convert a WMA file to a standard file format like WAV.

Once again, Audacity cannot read or write this format.