Catalyzes the hydrolysis of membrane sphingomyelin to form phosphorylcholine and ceramide. It has a relevant role in the homeostasis of membrane sphingolipids, thereby influencing membrane integrity, and endoplasmic reticulum organization and function. May sensitize cells to DNA damage-induced apoptosis. a sphingomyelin + H2O = an N-acylsphing-4-enine + H(+) + phosphocholine