I could see very specific instances where you would notice a volume difference. With say 4 Ohm speakers and long runs of crap-a-builder-put-in-your-walls 16 gauge wire, you could be adding 2 or 3% of the impedance in resistance in the wiring. Cambridge Audio says "resistance starts to have an effect on the performance of a speaker when resistance is greater than 5% of the speaker's impedance." So you aren't quite there, but you are close to there. Would that create an estimated difference in perceived volume of 10%? Hard to say, especially if your center channel has this effect and is a short distance or all your wire.

Bi-Wiring, if nothing else does reduce the resistance by half.

Other possibilities over length and gauge would be
- damaged wires in the wall.
- might you have not cut your wires back an inch or so to fresh copper? I don't know what resistance this would add or what, but I know with really high end ethernet setups you'd definitely be using newly exposed copper over reusing the old ends.