Why did the older and some current Black preachers preach and pray with a strange voice?

It stems from the black churches during slavery. Being a group in oppression and great tribulation and pain, the preachers were very dramatic to keep the people motavated and upbeat despite the racism and conditions they faced. The white slave owners used the bible to convice the slaves that they were meant to be slaves and that they were to serve their masters without any trouble. The horrors of slave and black life were relieved during church, there black people could feel loved and wanted by God. It became a place of sanctuary and energy and hope.