Better when it rains?

No, it is not your imagination. Your system is singing in the rain, just singing in the rain! What a glorious sound, you're happy again!
Seriously, back in the day, I noticed a deterioration of sound as humidity decreased. I don't doubt what you are experiencing is something similar.

Your system may sound better to you because of the added background noise the rain provides. For myself, I can't listen in a completely silent room. It just doesn't "feel" right. I usually have a small fan going on a very low setting. Just that little bit of background noise gives a more relaxed feeling to the room and system. I'm not sure why, but it works.

The effectiveness of your home's ground is in part a function of basically having most, if not all, of the right chemical elements (some 80+ different ones) present in sufficient amounts in the soil where your ground rod has been driven. But, it's overall effectiveness for how well your system performs rises and falls as those chemicals are actually in the presence of moisture. That's why, ideally, the ground rod should be deep enough to penetrate the ground water level. The US spec is generally 8ft, but often that ends up being insufficient in many areas. Arid climes are more difficult to get good ground in than wet ones and, with good grounding conditions or otherwise, often systems sound better after a good, 2 or 3 day, gentle soaking rain. Regards. John

Ivan_nosnibor is on the right track in my opinion. However this goes beyond your own home, to include the local AC grid which I suspect benefits from the additional moisture and improved grounding when it rains.

During the summer months the rain also tends to cools things off, suddenly reducing the load on the grid due to less AC demand.

I have no written proof of this but have noticed exactly the same effect on my system, not just recently but for many years.