Simple web alarm clock
This is not a complete review, just a few things that jumped out to me:

getID()

Looks more like "easier to write" than "easier to read". From current function name "getID", most people reading the code will assume that the function will return an ID.

If you would like to use jQuery as you can simply write $("#hour") instead of getID("hour"). The core idea of jQuery is to let you select (or query) DOM elements.

alarmSwitch()

If you use simple if/else instead of the switch/case, the code will be much simpler.

if(aSwitch.On){
...
} else {
...
}


Variable Naming

Simply naming the variable tomorrow instead of tomo would have been better for read

var tomo = false;// tomorrow.


If/else blocks for single line

Consistently using if/else blocks, even for single line statements helps avoid hard to find bugs

if( aSwitch.On )
{
    aOff.style.display = "block";
    sound.play();
}
else
    alert("error..");