Sun, who has a past of getting into troubles, was at it again in Incheon after helping China's 4x100 metres freestyle relay team beat Japan on Wednesday (September 24).
"The Chinese let their anger out tonight," he is heard as saying on a Chinese-language audio passed to AFP, the news agency reported.
"Honestly speaking, the Japanese national anthem sounds ugly," he added.
Japan's swimmer Kosuke Hagino said he did not heard what Sun had said. "I didn't hear what he said so it's hard to comment," he said. "But first and foremost we're all athletes and we have to respect each other. We also have to show human values so hopefully we can continue to compete in that atmosphere," Hagino said.
"Sun is no stranger to controversy, getting himself suspended from swimming for six months after police detained him last year for colliding with a bus while driving his Porsche without a license.
China's swimming bad boy previously had a public bust-up with his coach after he voiced disapproval of the swimmer's dalliance with an air hostess," AFP reported.
Yang won gold in the 400m freestyle at the ongoing Asian Games. He is the Olympic champion in the same event.