Audio hardware that has waveform, MIDI, and mixer capabilities typically uses one driver to access its hardware functions. While the driver has separate entry-points for each audio function, the driver can use a single DriverProc entry point to handle the system messages.
If the same hardware supports video capture as well as video compression, you can combine both of these functions into one DLL and use a single DriverProc entry point to service them. The shared entry point simplifies the coordination of the different functions.