To sleep soundly
Why if someone has a good sleep is an expression "a sound sleep" used? Is it because a sleeping person makes any sounds?

HI
No, It is not in connection with the noises the sleeper makes.

'Sound' has a number of meanings.
The first and main one is, of course, in relation to noise.

But another main meaning of 'sound' is when something is "Free from defect, decay, or damage; something in good condition."

Example 1 - "The ship was sound and should not have sunk" = The ship was in very good condition and should not have sunk

Example 2 - "He is a sound man... I really like him" = He is a great man... a wonderful man... I really like him

And in relation to sleep:-
"I had a sound sleep" = I had a wonderful sleep, with no disturbances and I am now full rested.

Go to http://www.thefreedictionary.com/sound for more examples.