FLV Audio Extractor is a totally-free application for Windows which you will be able to extract the audio from those videos with.
No restrictions and a good quality, that what FLV audio Extractor offers you. Choose the video you want to extract the audio from, add it to the list of extractions/conversions and wait a moment. Once the process id finished you'll have the audio extracted and formatted as an mp3 file and ready to use.
Finally we have to say that the process is not a long and boring but process but it only takes a few seconds to extract the audio from FLV files.