I don't think I have the low pitch issue when exporting to .mp3, but it sure sounds in slower bpm although maybe it's my fault, I don't know much about music stuff... I was wondering about the g r o o v e buttons as well (I made sure the bpm/speed gauge was at the center when exporting)
I've uploaded the .json and the .mp3 to google drive in case it helps
Anyways, GREAT tool though, makes composing easy and especially refreshing for noobs like me!