The Oxford English Dictionary gives a clue. The adjective 'safe' , as we use it,is first found in Middle English, the English spoken from the Norman conquest to the mid- to late C15. 'Sound' meant 'safe, secure, free from danger'.It too is from Middle English but , interestingly, this particular meaning is recorded by the Dictionary as current only until the mid C16.People, seemingly, stopped using that adjective on its own, with this meaning, then.So it may be that the two adjectives used together date from no later than this period, while one of them has since stopped being current.We do preserve old words when they are in phrases; 'hale and hearty', 'goods and chattels', 'without let or hindrance','proof of the pudding' come to mind; and they survive long after they fell, individually, from current use.