Converting analog stereo to digital stereo
What you are looking for is the protocol called "I2S", which is a streamed stereo (or more) digital protocol.

It uses a method called TDM - Time Division Multiplexing - to interleave the left and right audio signals.  Eg:



First it sends, as a serial data stream, the left channel data for one sample, then an IO pin changes state (from low to high) and the right channel data for the same sample is sent.  The IO pin drops back low again, and the cycle starts again.

By using multiple IO lines for sample channel selection you can effectively have as many channels interleaved as you like, but 2 is the "norm" for most systems.