Play a Flash file once when site is loaded
From Joomla! Documentation
Suppose you want to play a Flash file or other multimedia file when the site is first loaded, but you don't want the file to play every time the user navigates to the Home page. This tip will show you how to do this with Joomla! 1.5.
Creating the Multimedia (MM) page
First step is to make the front page. All browsers will look for an index page first, but the index.html page is the first amongst equals. So create an index.html page and embed your MM file within that. Then either configure your MM or add a link on the page to index.php. This will then navigate your users to the Joomla site. As this is Joomla Wiki I am not going to talk through all the details of creating a non-Joomla page.
Home Page Link
Without further adjustment, whenever your user clicks on the Home page link in the main menu, they will be navigated back to your MM page. No matter how good your MM page is, this will get very annoying, very quickly. The solution therefore is to update the Home Link.
To do this follow these steps.
(From the admin panel)
1 - Go to Menu Manager
2 - Click New Menu
3 - Give it a Unique name (I used Sub menu or something harmless like that)
4 - Give it an Title (suggest the same as above)
5 - Leave Description and Module Title blank
6 - Save
7 - From the top bar - Menu -> Main Menu
8 - Click the radio button next to Home
9 - Select Move
10 - Select Sub Menu (or whatever you called it) and move again
11 - At this point it would be worth checking your site to ensure that you have no Home item in the Main Menu, and the sub Menu is not visible
12 - Next we need to create a new Home link.
13 - Navigate to Menu -> Main Menu again.
14 - This time select 'New'
15 - Menu item type is 'External Link'
16 - The Title and Alias should be set as home
17 - The Link is set as 'index.php'. Save.
18 - Your Home link should now be at the bottom of the Main Menu.
19 - Menu -> Main Menu (last time I promise!)
20 - Move your link to the top.
and et hop! next time you click the home link, it should take you back to the Joomla home page and not the MM page you created.