http://audacity.sourceforge.net

Audacity is a sound editing program which can record, playback, and mix sounds or apply effects using a variety of filters. Edit your sounds using Cut, Copy and Paste (with unlimited Undo) and mix multiple tracks together.  It also has a built-in amplitude envelope editor, a customisable spectrogram mode and a frequency analysis window for audio analysis applications. Built-in effects include Bass Boost, Wahwah, and Noise Removal.  This full-featured program is fast and stable, and is also available for Linux and MacOS X.

You can your files in a number of formats, including WAV, Ogg Vorbis and AIFF, or use Audacity's native project format for high performance editing. 

NOTE:  MP3 support is available using the external LAME MP3 encoder (see below), which can be installed from here after installing Audacity. The first time you attempt to save a sound file as an MP3, you must tell the program where the LAME directory is, which should be in your Program Files directory, in a folder called LAME.

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