I was only thinking the other day about this. We had a few rainy days and the moist atmosphere changed the sound. I did wonder if it was my hearing changing in reaction to pressure etc, but it may have been both that and sound waves travelling differently.
The speed of sound is affected by temperature, but I’ve never thought it was discernible as far as audio quality goes. If indoors I suppose it could, in theory, affect the room modes of a given space leading to a change in room characteristics. Much like adding wadding inside a speaker cabinet, effectively increasing the size of the enclosure.
For cartridges they definitely need to be above 21 degrees C or so to actually perform well; tracking gets massively improved. In many cases the only time people get to experience this is in the summer.
For the digital side of the system I do not hear a difference, but then again we are talking about a temperature span of at most 7 degrees between the coldest and hottest its played at, and humidity between 35% and 50%.
I find on humid days all my systems struggle to perform.
Air pressure is trickier. The effects on sound are well dicumented. However, I lived on the 30th floor and my ears popped every time I rode the elevator home. My Naim system sounded great though. And a lot of audio manufacturers are based in parts of Colorado where the high altitude air pressure is very low but air is arid and dry.
When I worked in retail, a guy from Linn told us that materials like the suspensions of loudspeaker cones and of cantilevers on cartridges become more compliant at higher temperatures. So the cones and cantilevers accelerate and decelerate faster.
This seems the most plausible explanation. This summer is the first time I’ve noticed this, but it just so happens that this time i’ve got a relatively new pair of speakers in my system. Perhaps they are not fully run in yet (despite being 2nd hand) and the heat is loosening up the driver suspensions as you describe