Inputs and outputs can be connected to each other by means of software controlled switches. There may also be several volume controls, base and treble controls and an AGC control for the Microphone input.
Different boards have different architectures.
The switches, volume controls etc. are known as the mixer. These settings should never be changed once the correct settings for a specific hardware combination is found. If OSS is used, the program ossmix should be used to set up the mixer. I have a script running ossmix several times to set several parameters to what my hardware requires. Different boards may have different names for similar switches When you have installed for the first time and try to run the dsp program you will most probably have direct sound in the loudspeaker.