You're not being dense, not at all. I was in a hurry and not clear enough: Sound is different of audio signal. Audio signal is whatever way you arrange to rep

You're not being dense, not at all. I was in a hurry and not clear enough: Sound is different of audio signal. Audio signal is whatever way you arrange to represent signals in the audio range of frequencies and, usually, dynamic rang. Sound is a physical phenomenon caused by variation of pressure on air that we can listen to. So, the transfer was usually done by audio signals, transmitted electrically via copper wire (and, sometimes, magnetic coupling in some modems). You could, of course, convert these audio signals to sound by picking up the phone (or wiring the modem to the sound card), but this was just for you to listen to. The communication was not depended on sound itself.