Heir Audio’s youngest children have been thrust into the thick of a do-or-die competition. Custom earphone manufacturers are pounding with exceeding energy toward the lucrative – and showy – universal earphone market. I see no end in sight – and to be honest, that is a good thing. Technology handed down from top-flight customs is good stuff. Heir Audio’s 3.Ai and 4.Ai carry the goods inherited from their more expensive, custom siblings.