If you have access to all possible frequencies, there is an enormous amount of data you can carry. Each TV channel is about 3 Megabits of data per second, and a cable system carries nearly 100 channels. However, the telephone system is designed to only handle the frequencies of the human voice, and the maximum amount of data it can theoretically carry is 4000 bytes per second per phone line. Then data compression was added. In current use, the information actually exchanged between the two modems may be much more complicated and sophisticated than the data that either computer sees.
Sound waves are a regular variation in air pressure.
If pressure changes once, it is not sound. Bounce a ball and a much, much smaller pressure change occurs, but because the pressure varies up and down over time in a regular pattern there is a sound. Sound can only occur over time. Furthermore, the lower the pitch, the longer the wavelength, and the longer the time needed before the sound can be clearly observed. A telephone has a microphone and speaker.