This was an interesting experiment. Sorry that I didn't have time to read it until now, but I have a few comments which may be of interest.
(1) I work in a technical area and have had a lot of fun reading and speculating about the effects of cryo treatment. In my line of work, I have access to some simple liquid nitrogen cryo temperature apparatus. So I assigned someone to just do a simple test of resistance change. We were careful to use very slow cooling - reducing the temperature over about a week's time - to prevent thermal shock. We were also careful not to use plated metals which I have read can have undesirable thermal cryo-treatment effects due to different contraction/expansion rates of the plating and base metals during rapid cooling. We also used very slow temperature rise (about a week) again to minimize thermal shock. The result showed a small decrease (a few % for most metals) in DC resistance of the metals from such cryo treatment.
(2) Non-cryoed outlets may sound better than cryoed outlets of the same model if they both use plated metal.
(3) Realize that any such small resistance change may not be the cause of any effect that you might hear, if any at all. Many of the problems of electrical conductivity between metals in contact are not the result of resistance, but are due to barrier potential effects - the understanding that electrons naturally exist at different energies in different metals. When two different metal types are placed in contact (as with most connectors including AC outlets) non-linear effects arise due to this barrier effect which can be changed from changing the properties of the metals used.
