I have this same problem with .flv videos downloaded from Tudou (
http://www.tudou.com). I have previously been able to buffer the .flv file to my HD, then copy it to where I want to save the file, but it appears that they changed the format so now the sound doesn't work in VLC. Here are some things I've noted:
1) In VLC (latest version, 'h') there is video but no sound.
2) In WinAmp, there is sound, but no video
3) Using the IrfanView video player everything works fine (I just don't like IrfanView for watching videos)
So it appears that all the encoding is valid, it's just that VLC is not reading it properly?
I'm adding a link to a file I uploaded. The .zip contains the first part of a Top Chef episode I downloaded with the above issue:
http://uploadingit.com/files/685221_itrj0/7a.zip