Allow me to clear this up for everybody:
Andrea Libman voices both Pinkie Pie and Fluttershy. She does not sing for Pinkie... normally. Shannon Chan-Kent normally sings for Pinkie. But, in a few rare instances (this episode and Baby Cakes) Andrea herself sings as Pinkie. That's why it sounds different that usual, a bit like Fluttershy.
Appears on these pages
00:26 Pinkie Pie - Welcome Song