This may not be possible. If it is, I would reckon you can't do it directly thru a stringtable. If you can, it would most likely look like this:
class timeleft
{
name = "timeleft";
sound[] = {"ticktock.ogg",db-0,1.0};
title = TIME_LEFT;
};
Now in your mission, "TIME_LEFT" will be defined as a global variable of string type. So in the same script you call the radio command repeatedly, you would also want to format the string to be used (TIME_LEFT = format["%1 seconds left",_timeleft]).
If you wanted to, you could use the "localize" command to get the "seconds left" part of that above string.
Again, I'm not sure if this would work, but apparently you can use global variables in a similar way for different things inside the description.ext (dialogs for example).