This takbeer has become a forsaken Sunnah nowadays, especially at the beginning of these ten days. You can hardly hear it except from a few. It should be recited out loud, so as to revive the Sunnah and remind the heedless.
It was proven that Ibn ‘Umar and Abu Hurayrah (may Allaah be pleased with them both) used to go out to the marketplace during the first ten days of Dhu’l-Hijjah, reciting the takbeer out loud, and the people would recite takbeer when they heard them,