Flash supports audio elegantly. Including audio in your movie is a simple process of importing the sound and then deciding in which keyframe you want the audio to play. Many options are available on how the audio playsfor example, whether it plays and finishes naturally (Event) or whether you want it to lock itself to the timeline so images remain synchronized no matter what (Stream). You're also given sophisticated envelope controls for each instance of a sound used.
Because the effect on file size is the biggest "cost" of using audio, Flash provides a variety of compression technologies and settings to individually or globally specify the kind of compression you want to use. I'm sure you'll find MP3 or Speech to be the best quality, when considering file size, for almost any sound you want to use, but other alternatives do exist. If nothing else, I hope this hour has made you more deliberate and restrained when adding audio to your movies. Don't get me wrongthe power of audio is great. Just try not to abuse this power when you're given it. (Your slow-connection users will thank you.)