Catalyzes the hydrolysis of membrane sphingomyelin to form phosphorylcholine and ceramide (PubMed:25180167). 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. In skeletal muscle, mediates TNF-stimulated oxidant production (PubMed:25180167). a sphingomyelin + H2O = an N-acylsphing-4-enine + H(+) + phosphocholine Activated by phosphatidylserine and tumor necrosis factor (TNF) (PubMed:25180167). Inhibited by scyphostatin (By similarity). Expressed in skeletal muscle (at protein level). Expressed in skeletal muscle but a lower levels than isoform 1 (at protein level). Extended N-terminus.