Specific countdown timer

I can't make this work!
I need to display a countdown timer in a cell on a spreadsheet. (Merged cells with K3 the top left, format mm:ss)
The start time is loaded by entering minutes in cell O4 and seconds in cell P4, then RESET command button loads the value to the timer cell (K3).
The timer will count down by seconds. The spreadsheet needs to be active while the countdown timer runs, and when it is stopped.
I will use toggle switch named CLOCK to start and pause the countdown.
If I enter 10 in O4 and 00 in P4 and click RESET, the timer should start counting down from 10:00 when CLOCK is clicked.
At 6:33 if I click CLOCK again the timer should stop. When I click CLOCK again the timer should start counting down from 6:33.
When the timer is stopped, I need to be able to correct the time using the RESET button.
If the timer runs to 0:00 a sound event is called. I have the sound module (API) and
will use sndPlaySound"C:\file.wav,1"
Finally, I need to change the format and code to show tenths of a second when the timer value is less than 60 seconds.