It’s poetry, so it doesn’t have to really make sense — each listener can interpret it as he or she would like. I’d say it’s a play on words. There is an expression “safe and sound”, from one definition of "sound" (from Old English). From Google: in good condition; not damaged, injured, or diseased. Another definition of “sound” is from French / Latin. This definition may well apply, as the words are from a song, and it’s natural to think of the sound of a song or similar. Again from Google: vibrations that travel through the air or another medium and can be heard when they reach a person's or animal's ear.