Why is the quality of sound bad?

I have noticed that the phone sound regardless if I listen through the phone or have it on speaker has become less than great. For reference I have had this phone for more than 4 years and the problem began last year after a battery and screen replacement. Guys who serviced it are still reassuring it’s my idea. But it is not. I’ve had other phones around and checked that the spatial let’s say sound is bad only on mine.