Greetings,
If you open the Avast Mobile Security app on your phone, click on the three horizontal bars at the top-left of the screen, you should then see the list of apps within the suite, including the Anti-Theft one (third from top on my phone).
Click on the Anti-Theft option - it may then ask for the four-digit PIN - then deactivate the "Sound siren" option.
This should allow you to keep the security benefits without the embarrassing false alarms.
Kindest regards,
James