If there’s no sound at all, you’re probably facing a hardware issue. In that case check carefully with a good light source your lightning and headphone ports. If you can see any debris or dirt that has to be cleaned with fine tweezers, canned air. If it’s just some Apps having no sounds it’s probably a software issue. In that case you can try a full wipe through system Settings/General, scroll down to the bottom. Don’t forget to backup on Pc before restore if you have data you care for.