из хелпа по классу Sound:
Код AS3:
public function load(stream:URLRequest, context:SoundLoaderContext = null):void
Parameters
stream:URLRequest — A URL that points to an external MP3 file.
context:SoundLoaderContext (default = null) — Minimum number of milliseconds of MP3 data to hold in the Sound object's buffer. The Sound object waits until it has at least this much data before beginning playback and before resuming playback after a network stall.
The default value is 1000 (one second).