Thank you very much for your help.
The code broke my ajax loading. However, I found a solution via cookies with Jquery, but I realized that my idea was wrong, because as you said, it loads once ever, so tracks are not playable after one reload (and I want to keep the button play next to every mp3 link).
Here is my test site, that is more talking that thousand words (and I'm not english well speaker...):
http://www.actupost.com
In fact, I'd like the Yahoo Media Player, in the left bottom, not add duplicate tracks at every load. However, I'd like to keep the play buttons on pages.
To make tests, just click several times on the first post and you'll see the playlist rise at every load. May you have solution to not make the playlist rise indefinitely with duplicate tracks ?