How long are your cables? It's usually not an issue if you are going between buildings or something like that. With unbalanced connections within a house, ground loop noise is the most likely problem.
The relationship between voltage, resistance, and current is defined by
Ohm's Law. Resistance (and impedance, both Ohms) is
"the resistance to current flow".
Cables have a certain capacitance per-foot and higher capacitance makes lower impedance which makes a lower impedance load on the output, "pulling" more current, and potentially dropping the voltage at high frequencies. (Capacitive reactance is a component of impedance).
There is also some resistance per-foot and although it can be an issue with speakers it's not really an issue at line connection impedances. With a very-long run, you might get a fraction of a dB drop. And since the load is normally resistive, any voltage drop caused by resistance is flat across the audio frequency range.
You can calculate the output impedance by adding a load resistor to make a
voltage divider and then measuring the voltage drop. The "top resistor" is the built-in effective output impedance (not really a resistor) and the "bottom" resistor is the one you add as a known-load. A 1K resistor load is probably about right but maybe get an assortment of resistors and start with 5-10K.
This can be a bit tricky because you aren't supposed to "match" the impedance. (A matched impedance load would cut the signal voltage in half.) That is, although the output impedance may be less than 1K, it's usually not designed to drive a 1K load and you may not get enough voltage-drop for a good measurement. If the load impedance is too low you can get too much current (more than it's designed for) and stress the electronics, without showing much of a voltage drop.