Under windows I tried to use avisynth to extract video and audio from FLV files and of course you'll need flv codec (I believe I installed ffmpeg and I could play FLVs in media player classic)

There is also a freeware called FLV Extract that can split video and audio from FLV files without decompressing or recompressing. The video is saved to AVI and the audio is saved to MP3.

http://www.moitah.net/