There are two common types of files that carry the .fla extension. Firstly you might have a Macromedia Flash Authoring file. This can be opened with Macromedia Flash software for editing.
Another type of file you might have when you get a .fla file, is a FLAC music file. FLAC stands for Free Lossless Audio Codec. It is open source and provides lossless audio compression. This means that even though the audio is compressed, it will still maintain its exact original quality. However, since it is lossless, music stored in this format would definitely take up more space than say MP3 or Musepack audio or any other lossy compression.