Here is a link to the User Guide for your player.
If you are trying to use the Volume control on the side of the player (p.13 of the manual) to adjust the sound level and it is not responding, it may be that the control is faulty or perhaps there may be a wiring or component connection problem
Here is a link to the Ifixit guide on how to open the player so as to gain access to the inside of the player and then to the volume control, which should be able to be seen at about Step 3 in the link below, so that you can ascertain what the problem is.
Philips PD9000 37 Buttons Replacement
Hopefully this is of some help.