This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
We have been unable to find any formal standard on this topic.
However, there is an EIA Bulletin: EIA Consumer Products
Engineering Bulletin No 6-A (CPEB6-A) 1974, titled "Preferred
Voltage and Impedance Values for the Interconnection of Audio
Products". The key word in the title is 'Preferred'.
EIA CPEB6-A recommends 3mV at 47k ohms for magnetic phono
cartridges, 250mV at less than 10k ohms for tape and preamp
outputs, and 100k ohm minimum for tape, tuner, and amp aux
inputs. The bulletin also has information on microphones,
and headphones. You can order a copy through a technical
library or directly from the EIA.