Some sound files would be perfect instructional material if only they were not so fast: radio programs, songs or any other authentic material. You can easily change the speed or temp of a sound file using Audacity. The speed (tempo) of the speach can be slowed down without making the voices sound especially deep (no change in pitch). This change will of course lengthen the audio track.
The tempo tool will work on the selected audio. Select the entire track by clicking in the control box. With the track selected go to the menu Effects -> Change Tempo. A dialog will appear. Decreasing the tempo will slow down the speed of the speach. To decrease the tempo you will need a negative percent change or move the slider to the left. Once you have accepted the change the selected audio will be processed and a new track created at the new tempo.