The loudest mouth has shut up. Muhammad Ali is not talking to newspapermen. This is news of the man-bites-dog variety and a lot of people have guessed at reasons for Ali's sudden attack of silence. In Leon Spinks beneath spoken insult? Has Ali finally exhausted his hot air? Or, most sinister, is the champion on the outs with the Black Muslims?

That last possibility is intriguing to anyone who has studied the transformation of Cassius Marcellus Clay Jr. into Muhammad Ali. One theory, propounded mostly by white Christians, is that Ali is a Muslim today only because he once lived in fear of what would happen should he tell the Muslims to get lost; enlisted, Ali was made wealthy and even became a believer. Anyone buying that idea would find it reasonable that Ali would stop talking if the Muslims, upset with something, told him to cool it.

Something seem to be going on. According to reporters in Las Vegas, Ali's normal entourage of Muslims arrived late for tomorrow night's championship fight with Spinks. And for unexplained reasons, Ferdy Pacheco, the white doctor from Miami who has been in Ali's corner for every title fight in his grand career, will not be working for the champion this time. In these developments are the seeds of wonder.

The fight is hardly worth mention. Although Spinks has an impressive amateur record that includes the Olympic lightheavyweight championship, he has been a pro only 13 months. He's undefeated, winning six times and getting a draw with Scott Ledoux, nationally famous only as the guy who knocked Howard Cosell's toupee sideways while claiming fix in the Don King-promoted tournament.

Perhaps senility will set in five minutes after Ali laces on the gloves tomorrow night, but if it doesn't, Spinks has no chance. He's a good puncher, certainly, and that would help him against a fighter who can be hurt by a good punch. Ali has taken the best of Frazier and Foreman and Norton. Only a smart head can beat Ali. Dogged research shows nothing in Spinks' record to suggest he has the necessary schooling to outwit the most intelligent ring master in four decades.

One vivid example suffices. By March 20 of 1977, Ali had fought for the heavyweight championship an astonishing 20 times. On that day, Leon Spinks fought the third pro fight of his life. His opponent was a bartender.

"I told 'em I wasn't in no shape to be fighting," said Jerry McIntyre, 26, who runs a bar called Water Hole No. 3 in Harker Heights, Texas. Although he weighed 232 pounds and hadn't trained in five months, McIntyre took the promoters' $1,500 and went in against Spinks.

"When a man's flabby," Spinks said after the fight, "you look where the flabby's at and you hit it." So Spinks hit McIntyre in the belly a few times. Then he hit him in the face. The fight was over in 35 seconds.

McIntyre, assessing the damage, said, "He's a real good puncher. He smothered me with punches."

Then he took hold of his nose.

In 35 seconds against a bartender who has "Cynthia" tatooed on his right shoulder, you do not learn anything that will help you beat Muhammad Ali, even an Ali troubled enough to stay away from the attention that for so many years has been the driving force in his life. Make no mistake. Without the constant attention of newspapermen and TV-radio types, Ali would have withered away. That's why he's fighting now, well past his prime, because he has tasted the fruits of celebrity and found them intoxicating.

That's one man's idea, anyway. Others see it more darkly. They see the Muslims needing money and a forum. Both have been supplied by Ali, who will answer a reporter's every question except those testing his financial and religious ties with the Muslims. On those questions, he filibusters, parroting sermons. Who controls Ali so tightly? Who . . . or what? Some believe it is fear, and that belief has a foundation built on the assassination of Malcom X.

Fifteen years ago, when Elijah Muhammad led the Black Muslims, his favored protege was Malcom X, then a foundation of rage who shouted from every street corner that the white man was a blue-eyed devil. Shortly after Cassius Marcellus Clay Jr. won an Olympic gold medal, Malcom X won him to the Muslims by the power of his oratory.

In 1964, the day after winning the heavyweight championships, Clay announced to the press that he was a Muslim, Muhammad Ali. His teacher, his proud recruiter, was Malcom X.

Then Malcom X changed. When Elijah Muhammad's secretary filed a paternity suit against the Muslim leader, Malcom X denounced him a hypocrite who preached purity but practiced adultery. After a trip to Mecca, during which Malcom said he had a vision of all men at fault than the black. Malcom X was ordered out of the Muslims.

While working on his autobiography, Malcom told writer Alex Haley that the Muslims would assassinate him for speaking out against them.

Before the book was finished, men with shotguns killed Malcom X.

Muhammad Ali will not talk about Malcom X.