T.S.
She sounds totally normal to me. I think sometimes when kids' speech is advanced, they are talking more than their little mouths are ready for and they can't quite make all the sounds. My son is almost 2 and a half and talks a ton, but so did I (actually I still do). He has all kinds of sound substitutions.
I think if he talked more like a 2 year old... using only a few words in a row... you wouldn't notice the issues because he would only make the sounds he is good at. By the time I was 5 they were all gone. I had tons of subsitutions as a toddler and they were all handled by the time I was 4 or 5.
As for the ABC song... I wouldn't worry. Sounds like she knows the song but not really how it is connected to the letters. When she gets to the tough part to sing, she skips over it... no biggie. The two year old you saw probably knows the letters so was actually thinking of each letter as he/she sang. If you're worried about it... ask her to sing "L-M-N-O" I bet she can... she probably didn't know it was meant to be those letters.
If you're worried, ask her pediatrician, she can give you better advice than we can about what is normal.
T.