I’d probably use a chunky LiPo battery (or a USB power bank).
Check out this forum thread for discussion of adding a microphone: Adding microphone to Unicorns? (though from what I remember, calculating decibels requires some non-trivial calculations!)
There’s some cool web projects here: pimoroni-pico/micropython/examples/galactic_unicorn at main · pimoroni/pimoroni-pico · GitHub