I have had a similar same issue on wireless headphones, it seems to be some sort of fade in on the headphones which happens when there is no sound and then a short sound.
A trick to solve this has been to put some very quiet lofi as a background to have continuous sound output and then the problem disappear.
Since I mentioned Anki audio autoplay + headphones earlier, this fixes the problem for me too. I don’t always put on headphones or earphones, just when I don’t want to bother others. I’ve heard about the solution, but never tried, rather prefer almost silent, just barely any insect/traffic noise.
Since the OP mentioned speakers too, not sure if it the same problem. It may have to be autoplay, plus enough consecutives. (Does it mean, not all, just 3 consecs and sporadic?)
Nonetheless, it can’t hurt to try a quick fix, as it may help diagnose the problem.
This may be related to a longstanding issue between Firefox and Bluetooth audio. If you’d like, feel free to email us at hello@wanikani.com with your setup details and the devices/browsers you’ve tested so we can look into it further!