I was watching the scene during the end credits of season 4 episode 20 "Pulling Teeth," and heard her voice. Here's the episode: http://www.thedarewall.com/tv/family-matters/season/4/episode/20

Were there other episodes in which we heard his mom or dad's voice?

Yeah thats the only time we get to hear the voice on the show i am thinking the producers were thinking about adding steve's mother on the show but change their minds.