Better sound on rainy days?

Comments about sound in water and whales are irrelevant, unless you and your system are fully submerged.

Air pressure influences our ability to hear.

Depending on the amount of rain, the effect it has on the ambient temperature will help with overhead power supply cooling and improved transmission efficiency, which is a factor in providing a cleaner power input.

Also, if you have a local grounding system, the resistivity reduces if the rain is sufficient to get down to the grounding. This can be a significant factor in revealing systems.

Your aircon and fridge is not working so hard in cooler weather = your hi-fi system gets a cleaner power input.

Psychologically, a dull day could focus our minds on the music more than a sunny day, which may offer more external distractions. Depending on where you live, you may feel more comfortable in a lower ambient temperature. Then your mood could be improved.

@mahgister / etal

With your car performance analogy, I suggest the air, or the O2, density is providing the largest influence. Hence the reason why some car engines are fitted with inlet manifold intercoolers. 

H2O injection was a big thing in motorsport several years ago, because of the release of O2 into the combustion chamber increased power output for a given engine capacity, until it was banned by regulation.

Interesting about -ive ions. They will play a part as actual humidity rises.

Cheers,

BP