Hi, after 2 months, I found a "2 hours solution" that works but with a small concern (I think).
I moved my iTunes folder to my Desktop.
I launched iTunes so it creates a new empty library.
I moved to this new itunes all my mp3 (not all at once, i put like 1000 mp3 per times), my ringtones, my videos from my original library.
It took me 1h to copy all.
Then I went to iTunes store, in "My purchases" (I'm french so I don't know the exact translations) and redownloaded all my purchases (apps, music, videos) in 1h.
Then all my mp3 played ok.
Concern : after this "migration", I don't know why, but when I download updates for apps in iTunes, these updates are not transfered to my iPhone when I use the sync button in iTunes. Previously, everything was transfered without problem.
Thread for this point : iTunes doesn't tranfer updated apps to my iPhone