First of all Raj won't be able to recognize Jassi's voice because he's a guy. Men don't take notice of these things. They only pay attention to someone's looks.😆
Second, never in his wildest dreams would he think Jassi would look as good as she does. When looking at some of the pics of Mona (before the transformation) I would have never guessed she was playing Jassi - and that is real life. So there is reason to believe that Raj and everyone else would not recognize her. And as for the voice, unless it's a distinct voice as in very high or low, it's hard to tell. I've seen animated movies and with voices of actors and actresses and sometimes I can't figure out who's voice it is until seeing the credits.