Back in 2003 adicoto listed a sum of codecs to decode different streams. As for Flash videos here's the extract from
his post:
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Flash video - FLV playback (FLV1 and FLV4):
1) First install FLV splitter:
SourceForge.net: Guliverkli2: Downloading ...
THEN
2) install FLV decoder (NOT needed, if you already have FFDshow installed)
ffdshow tryouts | Official Website
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As you can see (and as I remember) there were only a ffdshow decoder at that time with the (still actively developed)
guliverkli splitter.
I think you have to make ffdshow to work, to be able to play flv files