My opponent seemed to me (then a teenager) an elderly guy but I think he was just in his 50s or 60s.
Anyway, he literally was audibly and somewhat loudly talking to himself, saying, "If I go there, then he goes there... but then... and I got him!" -- even in a casual game, much less in a large and crowded tournament, this would not be acceptable.
He was older than my dad and he was losing anyway so I did not say anything. Then another player, also eccentric looking and acting (I knew of this guy, a "B" player who called himself a pro -- almost impossible because if he did manage to win a class prize, he would move up to "A" anyway: I think only a GM, in those days before the Internet allowed pretty girls and articulate FIDE masters etc. to make money from Youtube) came to our board and, fairly loudly himself, asked, "Is there something wrong with you??"
My opponent looked somewhat offended but said nothing and then significantly did shut up and I wonder if the whole thing was some sort of act trying to distract me.
But the guy who had chastised him then stared accusingly at me before walking off, as if to say, "Why are you making me do this?"
I am pretty sure I had no special responsibility -- I doubt if this specific thing is mentioned. I am sure there is a "silence" rule but I doubt that the opponent is supposed to be the first line of defense.
However, maybe informally I should have been the first to gently shush him.
This does not come up much in adult events (never played in a kids-only event).