Or if a mono font is too limiting - any font that supports the CSS rule `font-variant-numeric: tabular-nums` can be made to behave like a fixed-width font in this situation.
I have a (malfunctioning) beep but it doesn't work on mobile right now, I have to look into it. If you tried on desktop and it didn't work for you could you share what browser you used?
For the timer you should use a mono font so it doesn't jump around so much as it's ticking down.
A sound between stretches would be helpful for the times I can't see the screen. Would be good to be able toggle off/on.