I haven’t done this challenge so I’m just speculating here. Your clock definitely goes down to 0, and I do hear the beeping, but the beeping doesn’t start until almost a second after it reaches 0. Maybe this delay is fooling the tests?
Did you figure it out? I am having the same issue in my project and am looking for answers. Just like yours, my timer appears to work and beep when I use it, but the test is failing. The only difference is that I ended up using Audio() since I couldn’t get the <audio> tag to work.