Digital signals are transmitted by varying electrical pulses.
These then have to be converted into analogue signals for the human ear to be interpreted as sound pressures.
Your modern telephone has a digital to analogue system, first to transmit the signals and then convert them to be heard by the recipient.
Analogue signals are transmitted by varying electrical frequencies...(wave-forms), and reach the human ear as sound pressures.
Early radio transmissions were always analogue signals.
Sound travels through air as sound pressure waves...
...in space, no-one can hear you scream..!!
..Here endeth the first lesson...

...!
LOL...!
Paul.
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.