Hi @naque
Could be just a faulty Volume Up button contact (dirty or worn out)
Here's the [document|42071|service manual] that will help.
On p.5 it shows how to electrically test the Volume Up key to see if it is working OK or not.
If not on p.24, part #157 shows the part numbers for the volume button. Search on line for appropriate to your mp3 colour, ''part number'' to hopefully find suppliers. I couldn't when using this method.