Beginner's Guide to Adobe Flash/Sound/On the Timeline
Another widely used option for adding sound to a Flash presentation is by adding continuous background music or narration. This sound file would be placed on its own layer in the timeline. You would just need to decide when you want the music to start and then place the file at that point on the timeline.
Another option is choosing Event from the Sync menu. This will play the sound in its entirety at the first frame of the timeline. Once added, your sound will be visible on the timeline in waveform. (see figure)
If you need to stop an event sound you can do this by placing an instance of the sound at the specific keyframe in which the sound should stop and select the Stop Sync option for the keyframe. This will also stop all instances of that particular sound.
In a Flash movie, several sound layers can be played by the Flash Player at one time. Flash acts like a sound mixer by mixing the sound layers as separate channels. It is important to keep this in mind when adding multiple sound layers, as this may greatly increase your file’s size and slow down the performance of your computer.