Welcome to the Forums! Looking at the panel connections, the USB to Host connection is for tranferring MIDI data to/from your computer, there is no audio on that USB connection. The USB to Device is for connecting USB memory drives for saving/loading. The phones/audio out should work fineconnected to your microphone input on your computer, the reason for your lack of signal could be an impedence mismatch (i.e. output impedence does not match computer input impedence), the solution would be either changing input impedence on your input/soundcard (possible through settings on some computers/soundcards), or first going through a device which will give you a mic level rather than a line level. Also check all audio connectors and cable (including any adapters) to make sure they are working. Try troubleshooting with this info and let us know what you find.
DX7IIFD, SY77, SY99, Hammond C3, Steinway L, CP300, AW1600, etc.