Audio problem: When Tom Skerritt first "interfaces" with Mother after being awoken, he says, "Good morning, Mother" - it is not even close to being Tom Skerritt's voice. According to the book 'The Making of Alien' this line was dubbed in by Ridley Scott as Tom Skerritt wasn't available at the time.
Audio problem: When the crew are dining together just before Kane meets his grisly end, the scene cuts from a long shot, where Parker is saying something, to a closer shot, with the conversation continuing as before. The problem is that Parker's voice can still be heard, but his lips aren't moving.
Audio problem: When they are looking at the molecular acid eating through the decks of the ship from the facehugger, Parker says the words "Take a look at this man!", but his mouth is saying something completely different and much shorter.
Answer: If I'm not mistaken, the scene you're talking about (where Dallas consults Mother before going into the vent) was removed from the Director's Cut version of the film (which did get its own theatrical release in 2003). Perhaps that's the version they showed. I couldn't find the scene in its entirety, but is this what you're referring to? https://m.youtube.com/watch?v=OaoQES6C9ok.
Bishop73
Yes that was the scene. It was longer of course.
Gavin Jackson