From what you just said, it sounds like you have the Yellow (video out) from the DVD going into a Yellow (video in) on the Dell. Then you must have the right/left audio out from the DVD player (red/white jacks) going into a right/left audio in on the Dell. As long as these audio in's are going to the same set as the video in you shouldn't have a problem.
If the main problem is that the DVD sounds lower than the cable box, this could be caused by the fact that cable boxes have a setting to let the volume level be affected by the volume on the cable box, in the cable box set up menu, there should be something to let you set the sound to "fixed" output, thus the cable box volume control will have no effect, it will only be the Dell volume that raises or lowers it.
If this isn't the case, you may want to switch the audio cables from the cable box to the DVD and see if there is any difference, maybe you have a bad set of audio cables.
The coaxial or optical outs from the DVD are only going to be any good if the TV has the same type fo inputs, which most don't.
good luck