FLV Extract:
http://moitah.net/VirtualDub should also be able to do it, provided you use one of the newer versions that accept input plugins (that functionality is provided by the FLV plugin).
FLV actually just uses MP3 streams, unless there's been an update to the standard which allows for another format (like AAC, most likely).
AviSynth can do it as well. If you can play the FLV in WMP, then DirectShowSource is more than capable of handling it. FFmpegSource probably can too, but I've never tried.