Hi Rainer
Looks like the problem isn't with the JWPlayer but with the url itself. You can try it just by accessing it via brower.
You shoould insert url like:
~/media({KeyPath})
So xsl function will generate something like ~/media(MediaArchive:5210a7b7-c595-47f5-a3d8-a45cf62f9c97), which will be converted by PageRenderer class to a url like:
[web application path]/media/5210a7b7-c595-47f5-a3d8-a45cf62f9c97/fsdsf/DFdf/myfile.swf
As a result the file should be accessible by browser, if it still won't work, that would likely be problem N#1 I mentioned, and then you should build a url with .swf in the end:
/media/{Id of the media}/media.swf
