Aviary Audio Editor is a free online audio editing tool that allows you to modify and edit audio file formats. You can slice, cut, combine, change volume and pitch, and more. You can also add multiple tracks each with different settings, import files from your computer, and save the resulted audio files to your hard drive.
The tool can be fairly simple or very complex to use depend on what you want to do with it. It can be used by anyone, from novice to advanced.