Fixes for the transition from sleep <-> wake
- Only turn the transistor on when we are actually going
  to use it to prevent the extra current being drawn every
  time the WDT wakes up the chip.
- Make sure the buzzer pin is driven low at the end of the sound
  loop so that the piezo is always physically in the same state
  when no sound is made and transitions between sleep and wake don't make
  an annoying chirping sound.
- Tune the off time to compensate for the fact that the unit runs
  at 3V and not the 5V at which the WDT period is spec'd