Actually, here's a fun fact: the audio "chirp" that you have heard in association with black hole mergers is not slowed down. The measurement is a gravitational wave, of course, not a sound wave. But when it is played as a sound wave, that's real speed, not slowed. This is actually mostly determined by the frequencies at which the gravitational wave detectors (LIGO and VIRGO) are sensitive, which happen to coincide with acoustic frequencies that your ears can hear.