J.H.
Oh my that's wonderful! My mom didn't hear mama from me till I was nearly 3 (and for the same reason!) so I kinda know how you feel from what she has told me about the first time she heard it. (My little brother said it shortly after I did.)
I was at work and my babysitter called. She had never called me at work before, so I was worried. She told me she had something she wanted me to hear. She held the phone up to my first born (my third child now) and she's just babbling away and I hear my sitter say "It's M.." and suddenly my 11 month old says "MAMAMAMAMAMAMA" I started crying! It was the most beautiful thing I'd ever heard.