Audacity is freely available software which can be used to record and edit audio files.
This blog post includes a number of resources which should help you get started with Audacity.
Online training session
Clicking on the following link will play a recording of an hour long basic training session on useing Audacuty
Click the images below to see different parts of the Audacity Screen
Examples of audio edited with Audacity
Fade in and Fade out (from the effect drop down menu)
Changing the volume with the envelope tool
Multiple Tracks. Positioned using the Time Shift tool and volumes modified using the envelope tool
Download the latest version of Audacity for free
Download a range of aplications, including Audacity, which can run directly from a USB Stick.
You will need to download the LAME file from this site in order to create MP3 file
Online Guide to Audacity
Printable Audacity User Guide