In both Old English and Old Norse, 'sund' meant swimming, sea or strait; hence, it is now used to mean the last of these...a narrow sea channel, as in the Sound of Sleat near the island of Skye, for example.
Cheers, J2B. It'll be interesting to see whether someone comes along with a wrong answer and gets a 3-star rating! I'll manage to live with the rater's opinion somehow, though, given that my explanation of the word's source and meaning is fully supported by The Oxford English Dictionary - the 'bible' of word meaning and etymology.)