I was trying to use this website: https://guitar-tuner.appspot.com
But it wasn't working at all even after I allowed it access to the microphone.
I suspected this may be to do with me having six different "microphones" (input sources—I have both soundflowers, a device for my TV, and a bunch of virtual ones from Loopback)
I thought maybe Safari was using the wrong microphone, so I went to these two sites to check that and neither worked (they didn't even prompt for microphone access):