It's fairly simple. You go to File> Import to Library and then navigate to your sound file. It should be no higher that 44khz. I have found aiffs to work best because Flash in some cases does not like wav files and Mp3s are too compressed and can cause issues. Once you select your sound, you import it. You make a keyframe on the timeline, and select that keyframe, then go to the Properties box which if you don't see you can find by going to Windows> Properties. Under the properties Box with a keyframe selected you will find options to select a sound. Make sure that your sound is set to Stream and NOT Event or you will not be able to hear it as you scrub the playhead. That's pretty much it!