* warning: extremely technical post *
Yes, I know that the cheering can only be heard in explorer. This is
not for lack of wanting it to work in firefox.
Turns out that firefox will not let a sound event be activated by javascript (which is how the timer works) for security reasons. There is a workaround for this - dynamically writing the sound object to the page only when needed - only this takes ages to load - which totally defeats the purpose of adding it in the first place (the sound comes in way after time is up).
So, those of you wanting a silent timer, go for firefox (or turn down your speakers), and for sound, use explorer. And if firefox starts letting us use javascript functions of sound objects - please let me know!