Ok after looking through several forums and messing with filter (LAVFilters etc)/splitter and so on I found a new mediaplayer that seems to be totaly awesome and even better than KMPlayer:
PotPlayer:
http://www.videohelp.com/tools/PotPlayer
It plays damaged flv perfectly with sound and seekable where no other player could do that. Super customizable. Very happy I found it.
You have to download it on the site above, the original developer site only has a korean version.