Audio problem: On the moon, Superman smashes Nuclear Man's head against his knee. The clonk is heard before the hit (even forgetting that sound can't be heard on the moon).
Audio problem: When Lenny drives his car into the quarry, the sound of his radio changes depending on where the car is; but when he drives away, suddenly it stays constant. It should have decreased, and increased when we get to the close-up.
Audio problem: When Lex is giving instructions to launch the missile, a young man puts on his earphones and talks to Lex, but his mouth doesn't move.
Audio problem: When Nuclear Man knocks out Superman on the moon we hear some growling, but their mouths aren't moving.
Audio problem: When Nuclear Man puts Lenny down on the floor, Lenny babbles some things but his mouth doesn't move.
Audio problem: When Superman is wounded on the neck, and when the italian villagers are saved, the same male falsetto voice is used to add a (crappy and out-of-sync) "Oooh!" and "Aaaah!".
Audio problem: When Lois and Clark jump off the terrace, she cries his name. In the wide angle she moves her mouth but no sound is heard.
Audio problem: After the satellite crashes against the cosmonaut, inside the spaceship a Russian cosmonaut starts to cry out things, but his mouth isn't moving.
Audio problem: When Lex enters the military bunker, officer Goram's voice is obviously dubbed and sometimes the audio doesn't match his lips.
Answer: There appears to be multiple reasons. They had creative differences, ultimately resulting in a poorly received movie. Kidder said Reeves, who co-wrote the story, had an inflated ego and clashed with Furie.
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Moreover, Mark Pillow, who played Nuclear Man, claimed that working with Reeve was quite intense.