First of all, we Have Digital Computer so there is no sense of Processing an Analogue Signal. So Study of Digital Signal Becomes Necessary. Here it is...

Digital Audio Signal: Digital Audio Signal is Just Digitized, Discrete, Sampled and Quantized form of Analogue.

The very Basic form of this Analog-to-Digital Signal Transformation is: PCM- Pulse Code Modulation. One of these are LPCM- Linear PCM. You can visit Wikipedia for more detail. I am explaining it a little. viz