Extracting audio from flash (FLV) files
There's a slow method using VLC which also provides a fairly easy way to decide to switch between codecs. The menu seems to vary a little between platform but what you need to do is find the open/convert option and then follow the instructions.
On the command line batching is quite easy with ffmpeg - but by default some codecs are not supported which can make life a bit more complex. A fast approach which keeps the codec from the flash file (a fast option as there's no re-encoding):
$ ffmpeg -i input.flv -acodec copy output.something
You can investigate the file to start with - look at the output of ffmpeg without output parameters set or use your favourite tool:
$ ffmpeg -i input.flv
...
Duration: 00:28:05.01, start: 0.000000, bitrate: 292 kb/s
Stream #0.0: Video: h264, yuv420p, 480x360 [PAR 1:1 DAR 4:3], 193 kb/s, 29.92 tbr, 1k tbn, 59.94 tbc
Stream #0.1: Audio: aac, 44100 Hz, stereo, s16, 99 kb/s
At least one output file must be specified
Or you could look at the 'output.something' file and see what came out. 