Audacity is a free, open source audio editing tool. It allows you to create your own audio files or edit existing files. There are a number of built in effects that add to the usefulness of audacity.
You can download Audacity from: http://audacity.sourceforge.net/download/. It is available for Windows, Mac, and Linux. After editing the audio file, you can export it as a .wav file or an .mp3 file. The edited sound file can be used in Flash, PowerPoint, and a number of other applications.
If you would like to export the file as an .mp3 file, you need to also download an additional file (lame_enc.dll). Directions on how to install this as well as a link to the required file is available at http://audacity.sourceforge.net/help/faq?s=install&item=lame-mp3.
The video below will assist you with using Audacity to edit a sound file.