Re: Studio XPS 1640 Speakers not working...
There is no way that I know of to disable one jack. On some models one can install an inappropriate driver that does not recognize reconfigurable jacks at all and thus disables all of them including the mic jack and then the spealers work again. I know that is not what are after and I don't know of that kind of driver for your model anyway.
What has happened is that the sense pin in the jack has become damaged and left your computer permanently configured as though a plug is in the jack. The jack has to be replaced. If your model has a jack daughterboard it will be replaced. If not then the motherboard has to be changed out.
I don't have any advice about getting better service from Dell.