From http://www.w3schools.com (Copyright Refsnes Data)

Flash Sound

Example

Step 1
Choose File > Import to import a sound file. Locate the sound file and click Open. The sound file will be saved in the Library.

Step 2
Click in the first frame of "Layer 1" in the Timeline. Choose Window > Panels > Sound from the top menu.

Step 3
In the pop up window (Sound) choose the sound you imported in the Sound field. Choose Stream in the Sync field. Close the pop up window.

Step 4
Go to frame 50 in the Timeline. Right click and choose Insert Frame.

Step 5
Choose Control > Test Movie from the top menu to test your Flash movie.