Q. In the mahamantra can you please explain to me why do people pronounce Rama instead of Ram?

A. Usually people do not pronounce it deliberately. Sometimes when they are intensely chanting then they may be chanting Rama in stead of Ram. However Lord is pleased by the endeavor of His devotees. It is said that chanting should be like a crying of a child for its mother.
Yes, when they are performing kirtan, it is many times pronounced as Rama. However if someone is doing it for the pleasure of the Lord then it is alright. If someone is pronouncing it to make a show then Lord is not at all pleased with that.