It Was A Side Effect Of His Suit
His voice wasn't his own as shown by the Darth Vader's Armor article on Wookieepedia:
The med droids that had made the suit had inserted the redundant breathing tubes low enough so that, with the aid of an enunciator, his scorched vocal cords could still form sounds and words through a voice synthesizer in the mask. Absent from the enunciator in Vader's mouth grill, which imparted a synthetic bass tone, his own voice was little more than a whisper. Though limited by what the respirator could do, Vader could still make his respirator approximate a laugh or a sigh.
He himself could barely talk, but assisted by the voice synthesizer and the enunciator he could speak, shout, laugh, and hum.
The Wookieepedia page about his armor in canon is definitely lacking in detail but both canon and Legends agree that the voice change is caused by the suit:
The helmet created Vader's menacing growl due to his natural voice being very weak from vocal-cord damage.