I recently had all of my angle valves replaced with new ones, along with toilet supply hoses. While the plumber was there, we checked for leaks and did not find any. About an hour later, I found a touch of water leaking from each angle valve. The plumber returned and tightened the valve connections, and again no apparent leaks. After he left, I again checked and there again, I found a touch of water at the wall pipe and angle valve connection. He told me it was sweating because of the new pipe connection to the old one. Three questions: 1) is his "sweating" answer correct? and 2) will putting sealant on the angle stops correction this "leaking"? and it doesn't stop, what can I do?

Somebody is feeding you a load, old to new pipes do not sweat. Sweating is a cold pipe in a warm space so condensations happens. This has nothing to do with old to new. If it was sweat it would be the pipe itself befrore the valve would get any condensation. It sounds as though he used compression fittings but he may have also used a threaded or soldered fitting. Do you know which he used, maybe check the bill if you aren't sure.