How can you input data and files into the computer?

When audio signals (music, speech, etc) are detected by your computer's microphone, an analog (or continuous) electrical signal is generated. Computers work with digital data, so...there is a device in most computers called an analog-to-digital converter. This chip (or part of a chip) has the ability to sample the analog signal, calculate its amplitude, and return the value to a program (which the displays it, stores it on disk, mixes it, or does a multitude of other things).

This hardware capability is usually built-in to a computer or is a part of a sound card (like a SoundBlaster, etc), while the program is usually something like a Windows' Sound Recorder, audacity, etc.