I had the same problem with downloaded FLV videos as you have, this was my solution:
- Get some kind of free converter (Freemake: http://freemake.com / FLV converter: http://flv-coverter.org)
- Convert to any other format. I used MWV or AVI most of the time.
- The new format will have sound enabled, even in MPCstar.
I am not sure if this is a FLV-file problem, or a problem within MPCstar. But if you follow the steps above your files will have sound.