These micro:bit protective cases by Proto-pic, have built-in battery packs for 2x AAA batteries, which is great to run longer projects than the MI:Power micro:bit portable battery and speaker will handle.
The acrylic case protects the Micro:bit board from shorting by raising it, while allowing access to all the pins, buttons, and LED matrix. It also includes a labelled on/off switch on its edge.
Click here for more information on the Proto-pic Bat:bit Micro:bit case and battery.
Check out more micro:bit accessories here.