Answer:
Something you carry while singing.
I know a word of letters three, add two and less there will be.
How many people are dead in a standard grave yard?
When is 1500 plus 20 and 1600 minus 40 the same thing?
What instrument can make any sound and be heard but not touched or seen?
A hole in a pole, though I fill a hole in white. I'm used more by the day, and less by the night.