Replace buzzer in microwave oven with Windows XP startup sound http://tim.gremalm.se/%C2%B5mute-a-microwave-oven-silencer/
You need this repository cloned to your PC: https://github.com/sudar/Arduino-Makefile
Follow its installation instructions and then edit this project's Makefile so that path to Arduino.mk is correct.
Then clone this: https://github.com/PaulStoffregen/FreqCount and move it to your Arduino IDE /libraries folder.
Connect the buzzer output to pin 5, speaker amplifier input to pin 11 and enjoy your sound, whichever one you upload!