Ok there are a lot of variables here. For the wiring, yes it is possible for the cable to effect the sound if the gauge is super thick, but how much is probably not something that is quantifiable. Some things that may effect this is the purity of the copper in the cable, length, gauge, dielectric, corrosion, etc etc etc. So again a lot of potential for various results. And why are you using such thick wire? I seriously doubt it is necessary or beneficial.
As far as your question about CD vs a DVD player again lots of variables. Are they being played by the same source? If not you may have a cd player that has more gain than the DVD player. Also material is recorded differently. Most "Pop" radio friendly CDs are recorded at the highest gain threshold that the format is capable of with about a 3 db change in gain which means it will sound loud and slightly more loud and that's it, just like the crap on the radio (a lot of music is recorded in this way which really sucks).
A DVD is recorded with a lower level and will have dramatic upswings in dynamics as the action in the movie requires. This is to allow Matrix style gun slinging battles to have more drama vs other portions of the film.
Either way there are a lot of reasons for such volume differences.
