To insert a sound into your animation follow the steps written below:

1. To insert a sound into your Flash animation that is stored somewhere
     in your computer go to 
                                           File > Import

2. That will open a dialog box. Locate the desired sound file and click 
    open in the dialog box to insert the sound file in your Flash animation.

3. This action will place the selected sound file into the Library.

4. Now insert a new layer above the other layers of your animation by
                                         Insert > Layer

5. Select the new layer and drag the sound from the Library on to the 
    Stage. The timeline will look like as shown below. Always insert the 
    sound on to a seperate layer.

6. To listen the sound test the scene or movie.

                   An important factor that should be considered is length of the sound i.e the time period for the sound to last. The total number of frames that the sound occupy should be equal to the number of frames that are occupied by the animation. The Total number of frames that the sound occupy, depend upon the length of the sound and frame rate of the animation /movie. 
 
                  The total frames that the sound occupy can be calculated by multiplying length of sound in seconds and the frame rate of the movie. You can set the desired frame rate by changing the movie properties. 

                  For example, if your sound clip is of 10 second and you have set the frame rate of the animation/movie as 10 frames per second (fps), your sound clip will occupy 10*10 = 100 frames. So you should have the corresponding animation for 100 frames. 
                 
                  There are other expert way of adding sound but those are beyond this tutorial. This tutorial will just help you to getting in the idea of adding sounds.