I used a software from http://nchsoftware.com

There are two free software: one is video converter, one is audio converter:
1) use video converter to convert flv to avi
2) use audio converter to convert avi to mp3

If I use audio converter to convert flv to mp3 directly, about 30% of time, I get bad mp3 files .

Note: don't download the "plus" version