that page does say that:
Being that we are using a 555 timer chip, the maximum voltage that a 555 timer can withstand is 18V.
Thanks @warren and @roberts_ecofarm! This information is really helpful, and gives a way forward.
I will dig through my boxes of extra parts and find a potentiometer, maybe also a battery case for a 9v battery, then remake this circuit. I'll write in with how it goes.
I also found recently that a more powerful battery makes it louder, which i guess i could've figured out from the comments above (lol) but, just to say simply, use a 9v battery for louder sound! 😆 📢