That might have to do with the HTML wrapper around the SWF. You might need to swap in the latest SWFObject as the wrapper.
Can you please explain me more briefly..as i am new to as2 & flex 1.5
Flex 1.5 is no longer officially supported and it has been a long time since I used it, but IIRC, you provide an MXML url in the browser and the Flex 1.5 server returns HTML content that wraps a SWF.
The current versions of Flex use JS code from the SWFObject project http://code.google.com/p/swfobject/
I believe there is a way to get the returned HTML content to use this SWFObject code, and that may solve your problem. Of course, the better path would be to upgrade to Apache Flex.