Here is a blog post explaining this issue.
Flash- z-
<param name="wmode" value="transparent">
Flash OBJECT EMBED
: , , . Window Mode Flash- , .
* window - movie plays in its own rectangular window on a web page.
* opaque - the movie hides everything on the page behind it.
* transparent - the background of the HTML page shows through all
transparent portions of the movie, this may slow animation performance.