I get a bizarre echo effect with FLV files on Safari

http://www.biomedcentral.com/demo/NSP/immunity.html

In Safari the audio can be heard twice, with a small time difference, creating a echo effect. When the movie is stopped, one copy of the audio continues (without echo).

This happens in various Safari versions on different macs. It does not happen in Firefox or on pcs. The same Safari browsers can play flv files on other sites without echo problem.

The player used here is FLVplayer_Progressive.swf from Flash, but this thing also happens in Wijering's flvplayer.swf (embedded)

Source of these flv files was a dv-stream, but I also tried a mov with h264 compression and it had the same problem.

I tried several settings in ffmpeg, but nothing worked (tried also On Flix, and Sorenson Squeeze, and Flash Converter).

What am I doing wrong?