So after getting in contact with the person that made the blog A Guide to MGA’s Magic Touch Ponies, they updated the blog with the last two ponies and found a name. Unfortunately there was some kind of packaging error so the name could belong to either of these Magic Music Ponies. I'm gonna do a little searching myself to to see if I can find anything, but for now I'll share these two ladies again with the name and character bio below. As always, info was found on the blog linked above.
Antonia
“Antonia Magic Music is the happiest pony in Crystalia. She loves to tell jokes and pull many pranks on her friends. You help her keep a joyful mood by brushing her mane. Listen! She sings a sweet melody to thank you! Every time she sings, waves of happy energy flood through Crystalia. Together you spread laughter and fun wherever you go!”
In the words of the owner of the blog, taken directly from there as well: "Now these next two present a bit of a puzzle, as of the only boxed examples I can find, there appears to be a packaging error as they both use Antonia’s box, despite clearly being different characters. Thus, I am unable to determine which one is the actual Antonia – and the actual name of the other character!"
I think it's a pretty cool mystery. It would be neat to be able to solve it. Even if it can't be, i still think it'll be fun to try!