Aside from @ragingloli‘s advice, which is apt to a point – and maybe more elementary than you need – volume can be controlled in many places.
That is, the player software (or hardware, in case you’re using a detached player) can have its own volume control, the computer has a volume control (as noted) and even the speakers themselves can have volume controls (or software for the speakers or sound mixer) downstream of the computer.
It’s highly unlikely that the DVDs themselves are to blame. (Except that if they were pirated disks to begin with, or recordings of recordings, then the original sound quality in the disk itself may be poor.)