Double move sounds in variants

I’ve started playing in the variants server and I’d say 50%-80% of the time I hear multiple sounds when my opponent makes a move. I hear the normal piece sound, and then often I hear a second sound, that’s like a high-pitched two-tone, rapid ascending sound (much like the game start sound, but like up two octaves?). I don’t hear it every move, and I can’t for the life of me figure out what causes it. I only have one browser tab open, and I hear it from Safari on iOS and macOS. Has anybody else hit this? What is it, and how can I disable it?

Disabling move sounds disables it, but also disables the actual piece move sounds that I’d like to hear during a game.