The first time she spoke, The Crystalling, the credits just listed her as "Twilight's Mom." In Once Upon A Zepplin though, she has a different VA and the credits distinctly say "Twilight Velvet." Same with Twi's dad.
How interesting. I actually never noticed the difference in voice. Probably, of course, because both parents have hardly ever spoken on screen. And if I recall correctly, the stream of Zeppelin I watched got cut off during the credits.