Audio problem: When Fabian Cortez says "Mark my words X-Man. Before this day is done you will beg me to let you join us," his mouth movement is out of sync with the words "is done." (00:05:40)
Audio problem: Beast's mouth is wide open when he says "Feet don't fail me now." (00:02:05)
Answer: Chris Potter, the original Gambit voice actor, did indeed quit the role in the 4th season. His last episode was The Phalanx Covenant Part 1 so it seems reasonable that Fox would limit the use of the character even though it was recast. It doesn't appear that Potter left the role due to animosity, he stated in an interview that he wished to play Gambit in the first live action X-Men film.
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