@Gargron can't resist the shameless plug for https://currentlyuntitled.design/soundflakes/ and the couple of sound packs @talon, @guilevi and myself made with a bunch of notification sounds under CC attribution. I would say that having unique notification sounds, preferably customizable ones, is also an accessibility feature pretty much for the reason you just gave. If I hear that iOS tritone I don't know if someone just mentioned me on social media, someone just scored in sports or there was an alert from my banking app. So having apps give unique sounds really helps triaging what's more pressing to look at. So I really hope you consider changing that default sound to something else, whether it's one of these, something else or we could make something unique just for the mastodon app if you like.