After longer period, values of electrical component inside walkman change. This will cause that speed will be incorrect. In most cases it is slower but it is not general rule.
Setting correct speed is simple. You will need:
1. Speed calibration tape (link)
3. Small screwdriver
Insert speed calibration tape to your player and play calibration signal (3000 Hz sine tone)
Install “Frequency counter” software from Google Play store and run it
Put headphone as close to smartphone microphone as possible
Take small screwdriver and turn with speed adustment screw until Frequency counter software shows value as close as possible to 3000Hz
Where is speed adjustment screw located?
Sometimes there is small hole in walkman case, where tou can insert through small flat watch-repair screwdriver
In most cases, you need to open the back cover and locate speed adjustment trimmer somewhere on PCB. Usually it is located close to motor.
Sometimes it is clearly visible, sometimes it is hard to find. But it is always there!