How can I emit a (loud) 25 kHz sound wave from a Raspberry Pi?
GPIO pins aren't designed to drive high power loads, just provide data signals.  You need some kind of amplifier - even a simple MOSFET would help.  There will be plenty of examples of GPIO pins driving MOSFETS on the web.
If you can hear it, it's not 25kHz.  A very small number of people might hear it as an extremely high pitched whistle, but most people can't hear 25kHz at all, no matter how loud.  You might be hearing vibrations at some fraction of 25kHz, though.