[low speed digital communication over an audio line]
[very basic and low speed]
I'll bite at assuming you want to add digital data to the existing audio without "modifying" the audio. This is not going to be "basic."
My thoughts about this are to put an active low-pass filter after the audio source, to limit the audio frequencies to <20kHz. Then have a pair of 40kHz oscillators "inject" bursts across the audio link, which are picked up by the other device. Another 20kHz lowpass removes these pulses from the audio before exiting. If microcontrollers are used to time the bursts and interpret them, along with a bi-directional protocol of your design, then this just may work. The capacitance of typical audio cable will likely prohibit long runs however.
The same could be done with sub-audible frequencies and high-pass filters, such as 10Hz, but then the bit-rate must be very low. However long runs would not be a problem.
This is a far easier approach than professionally-encoding data over audio, which likely uses a DSP.
Another method could be to inject a +/-DC offset representing a zero or one and filter it out afterwards, but this seems technically more difficult to filter and prone to errors depending on audio content.