The balanced line standard often comes up here when discussing XLR cables. Here is the standard and here is an article by Rane that provides additional context. Also, see Chapter 3 of this article by Bill Whitlock, of Jensen Transformers, Inc. on balanced audio interfaces.
The balanced line standard covers grounding of balanced lines but not the 600-ohm thing that sometimes enters these discussions. The 600-ohms was for POTS (Plain Old Telephone Service) for which maximizing power transfer by matching output and input impedances was an important consideration. With home audio gear, almost everything has a low output impedance and high input impedance so the cable characteristic impedance is not an issue, unless you are running cables for tens of miles.