countdown alarm: add audible alarm when the countdown timer reaches expiration.
Per default, the audible alarm is turned off, but the user sees a mute symbol. When the user activates the audible alarm, the browser will announce the nearing end by beep signals in increasingly shorter periods before its expiration (default settings: 60, 30, 20, 10, 5, 2s; these values can be changed via HTML). Once a user has activated the audio signals, these are preseveed between page changes via a cookie. On some browsers (e.g. Chrome), the audioContext settings are so restrictive, that the full reload requires a new activation of the alarm via a "user gesture" (i.e. clicking on the audio symbol). It would be not surprising, if some other browsers show different behavior, but the code follows the relatively new recommendations. The code was tested with Chrome/Chromium 83 and Firefox 77).