Warm up time for record playing

@fsellet, you would be wise to discount everyone else's hearing. I am just trying to offer a reasonable and very factual reason for the OP's experience.  https://en.wikipedia.org/wiki/Stapedius_muscle#:~:text=The%20stapedius%20is%20the%20smallest,in%20th....if everyone wants to believe something else that is their prerogative. The only other reasons I can think of that would make sense are; a tube phono stage that needs to warm up, but that usually only takes 15 minutes. Vibration of the cartridge's suspension generating heat softening the damper. This would lower the cartridges resonance point and possibly its interaction with the tonearm. It would not likely produce any changes in the audio range. So, we are back to the little stapedius muscle. We can give rvpiano a case of bilateral Bell's palsy and see if the effect goes away. He can pose for an indian death mask at the same time:)

@thecarpathian, not a bad idea but unfortunately not necessarily true. You have to factor in expectation bias. In order to determine if one item sounds better than another you have to be able to switch back and forth. You can only pay attention to one item at a time. You can not evaluate the bass and the treble synchronously. Your brain switches between the two. It helps further if you do not know which one you are listening to. Expectation bias is very real. Magicians use it all the time along with the power of suggestion. 

Frogman, what I say about the physiology of hearing is absolutely, incontrovertibly true. Just google Wikipedia on it. The ONLY thing that is changing is your ear. Why do you think they have warm up bands at concerts! Notice the warm up band never plays at full volume? In small clubs the volume is not as loud but I still notice I am a little uncomfortable for the first 5 minutes or so. Once I do get comfortable things sound much better. All the other explanations I have seen here are assumptions without any proof. This is how mythology gets started. 
Millercarbon, you are just joking right?

Chakster, it takes one to know one. 

rvpiano, Of course the improvement is palpable. Why would the microphone changing it's sensitivity and frequency response be any different then your system doing that. I know for an absolute fact your hearing does this. Read any book on the physiology of hearing. Your ears and brain are far more flexible than your Hi Fi. People just do not understand how their senses can be so affected by the environment.  There is nothing about us that is static. 
Walk into a very bright room from a dim one (movie theater) and you will start squinting. Eventually things settle down and you won't have to squint...because your pupils have stopped down and limited the amount of light that enters your eyes. Guess what!  Your ears work the same way.

1+ elliottbnewcombjr.

Your central nervous system accommodates to the environment in many ways. Attached to one of your little ear conduction bones (ossicles) is the smallest muscle in the body called the stapedius. As the ear is exposed to louder sounds the stapedius tightens damping the ossicles to protect the inner ear. This also changes the frequency response of the ears slightly. This is auditory accommodation and the reason why you always want to warm up your ears before listening at higher volumes. I will start at 85 dB and increase 5 dB every five minutes or so up to my usual listening level for jazz and rock around 95 dB. This protects your ears from the damage of higher levels. Going above 95 db is fun but you do not want to do that on a routine basis if you like your hearing.