Saying Rabbana lakal hamd Loud in Jamaat

Q: Can one say "Rabbana lakal hamd" loudly in Jamaat (congregation) Salaah?

A: It is sunnat to say Tahmeed or Rabbana lakal hamd softly, and this applies to jamaat salah as well as the munfarid (one reading salaah alone). Saying it loudly will not nullify the salaah, though it is contrary to the sunnat.
However, the mukabbir (person appointed to relay the Imam's takbeer when the congregation is huge) may say Rabbana lakal hamd aloud and that will not be in conflcit with the sunnah. This is a standard practice throughout the Muslim world.