A single object tag for Flash is impossible if there is support (your Flash feature works in) in both IE and other browsers.
I'm not all that familiar with Joomla. But the critical issue is the page's served source code. I can tell you what that needs to be if I can see it. Making it happen via Joomla would be up to you. For Joomla specific help, you would probably need to consult the Joomla docs and/or a Joomla forum.
If you want me to have a look at the served source code so as to advise you on what may be missing from it:
Please post a link to the page on your site that contains the problematic code so we can check it out.
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