A semiconductor device is labelled as a "general purpose audio NPN device." What is it?

The term "NPN" specifically describes the layered semiconductor structure (Negative-Positive-Negative) of a Bipolar Junction Transistor (BJT). Along with PNP, these designate the two standard types of bipolar transistors used in electronics for amplification and switching. Field-effect transistors (FETs) are characterized differently, typically being referred to as N-channel or P-channel devices rather than NPN or PNP. Thyristors and triacs are more complex switching devices that use four layers of material (PNPN). Therefore, whenever you see a component described simply as an NPN or PNP device, it is a bipolar transistor.