Imaam an-Nawawee (RH) said:
“… As for other than the Imaam, the Sunnah is to say the takbeer in a low voice regardless of whether the worshipper is following an Imaam or praying alone.
One should not recite or do any Adhkaar in the Salaat so loud that it interferes with the next person standing, even in a prayer that is meant to be said aloud.