Midi out, Audio in

This struck me as strange. I sent Midi out to an instrument that returned audio at the line input. I control the transport and the volume from Msc. There is no need to turn it down since the mixer presets are bypassed. (or maybe disengaged from the port) I had read a comment awhile back about having to turn down the playback. But that appears unnecessary. It looks as though the instrument is under the control of Msc.