Better sound on rainy days?

Might be that the rain in damping all the ambient noise.

Yes dampening would be correct. 

Rain is like a waterfall....The atmosphere become ionized...

Ionization act on the body of the listener and in the room physical atmosphere making sound waves traveling and interacting more " easily" in the room ...

«Raining also causes an increase of negative ions in the air; negatively charged particles that come about because of lightning and the friction between the rain and the air. These ions can significantly improve the nervous, cardiovascular, and respiratory systems, helping people feel more comfortable and relaxed.»

I own a grid of ionizers, three different complementary types in my room....With the low cost Schuman generators grid they are my main non-Helmotlzian devices for my room acoustic control... 😁😊 All that at very low cost and with a big impact...

Ionization help a lot to create an intimate sound effect like listening to an headphone but the sound is outside the head 😊 but ionization alone will not be enough, i used a foldable wood screen as a psycho-acoustic devices to work with speakers A and B for ear A and B loke a lense balancing direct waves and reflkected waves.... But it is another matter not related to the OP question...

If you think i am "nut" read this review about Mr. Ishiguro costly acoustic panel using tourmaline as ionization device...And Mr. Ishiguro is anything but nut in Japan audio circles...

https://positive-feedback.com/reviews/hardware-reviews/acoustic-revive-rwl-iii-absolute-acoustic-panel/

What are the audible effect of ionizing the room?

I will borrow the words of the reviewer, they describe better than i would have, the effect of ionization, even if here he spoke also about acoutic panels where the ionizing tourmaline is indeed  integrated...

«The panels do not modify the colors, but they bring them out, they do not add top or bottom, but better focus the edges of the sound spectrum. The sound is much more engaging with them and also more relaxing. Virtually everything is improved—from space, through color, to dynamics.»

This describe well the effect i feel with my ionization devices, i dont claim that my cheap devices can replace three panels costing near 8 ,000 bucks for the three but my ratio S.Q. /price ( under 200 bucks) is over the roof...

I wrote this to motivate people to create their own devices and experiments....

Before steamships it is said that whales could talk to each other 10,000 miles away or more. Engine noise ruined all that. So, sound in water make a difference.

Didn't hear what other signals may be coming to the OP's location other than FM . 

There probably are many opinions about this crazy idea. But, many years ago I experienced a similar observance. Back then my primary sources were vinyl, cassette, and FM radio. There are some theories about enhancement of transmission caused by weather conditions.

That depends on the source of the transmission.

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.