Garage Band tutorials on the Apple site  
Open a new Garage Band Project  
     
Save it to the appropriate location  
     
Find the music from your iTunes Library under the "Audio" tab, choose your song  
     
Drag it into the timeline
work area
 
     
Drag the playback head to the desired time and select Edit>>Split  
     
Discard the undesired portion  
     
Select track>>fade-out  
     
Adjust the little purple orbs as needed  
     
Select share>> export song to disk  
     

Select "mp3"

"good" quality

 
     

Save as "mysong.mp3"

Later, you will import this file into flash