Anyone can pray aloud in any public setting and even on public property. Where there is a problem is when public property is a tool to promote religion, such as the case of a prayer being recited over a PA system owned by the public or more specifically, a public school system.
The 1st Amendment provides for the free exercise of religion but it also prohibits government endorsement of a religion or any religion.
Here's my take on this - A prayer doesn't need to amplified electronically for God to hear it. If God is what/who we believe him to be, we don't need a public display of how good of a Christian we are: he knows what is in our hearts.