Hi klausisbruch,
if I understand correctly, you want to use former premium feature of Avast Anti-Theft to record some audio from the my.avast.com web interface.
To do that, you have to activate the premium license in Anti-Theft app, not Mobile Security, because in the new version that's not possible anymore. Open the Anti-Theft app (by dialing the PIN code you set up when activating it), find the "Go Premium" button at the bottom and then find "I already have a license" button, which should be on the purchase screen. This will initiate the licence check, which now gives everyone the license for free.
Hope that helps
Ondra