Hi Aurelie,
All of the webinars have been uploaded to Connect. Please login to Connect, then click on the link to either Ariba Contract Management ORAriba Buyer in the top left pane then click on the link to Ariba 9r2 Feature Preview Webinars under the Documentation heading. That will bring you to this page that list all of the 9r2 webinar recordings:
https://connect.ariba.com/AC_web_page/1,,154839,00.html
The link to the Workspace Audit Trail and Platform Compatibilities webinar is here:
https://connect.ariba.com/AC_Content_Details_Page/1,,2_154833,00.html
Hope this helps
-Glenn