You are dealing with differential inputs. The idea here is that the receiver of the audio signal ONLY sees the DIFFERENCE in the voltage between the high and the low connection. Any voltage that is COMMON is rejected.
We do this to reduce coupling common mode noise from one piece of equipment to another.
In principle, if we have a true differential audio input it does not matter which line does what - they would be equivalent. However in most cases we may share a connector pin between several inputs or our differential receivers have a limitation or we condition one of the inputs in some or other way so let's treat them as different.
The big point of all of this from a wiring view: Your receiver "lo" line is signal ground - the ground of the audio transmitting side - NOT your own ground.
So wire that to the audio signal ground of the other side unless the other side has a dedicated "lo" pin dedicated to its OUTPUT.
Do NOT make the mistake of wiring your receivers "lo" to a receiver "lo" on the other side.
If your transmitting side does not have any form of audio output or signal ground - use its power supply ground pin.
Typically in our aircraft audio systems we treat the "lo" as not having an actual audio signal but something that is audio ground "from the other end of the audio connection". In a way - it measured the noise there relative to its own ground so it can subtract it from the signal.
Rainier
CEO MGL Avionics