Just compared my Piezo buzzer from this kit, https://shop.pimoroni.com/products/sidekick-basic-kit-v2 with the thin mini speaker I have and loudness wise they are about the same. The speaker definitely sounded better but wasn’t really any louder. Not that my ears could tell. I can hear it quit plainly from a few feet away though.
music.play(music.BIRTHDAY,pin=microbit.pin0, wait=True, loop=False)