Is it in your PC or part of your TV set up. If it's in your pc go to: Start > control panel > sound speech audio devices > adjust system volume. If it's part of your TV set up then I would guess that you will find a volume level somewhere in your DVD player on screen menu.
Hi
Then I'm as lost as you are, Have you tried a google search for your make of DVD to see if other users have an issue with the volume control? Iv'e got a cheapo Yamada DVD player but it does have a volume contorol button and does everything it says on the tin. Sorry I can't be of more help.