Was your DVD player playing DTS Audio fine before? If so, and it has recently stopped, I'd check if there are any driver conflicts for your laptop. Check the dell site and see if you can download updated drivers for you machine.
Next, I would check the DVD player software, if it came pre-packaged from Dell then you are probably running a limited version of Power DVD. Check online and download the full version of the product. The full version gives you the ability to select the sound stream from within the software and not by the DVD.
Also trying google groups for information. If it is something common you may find somebody who had the same problem as you.
Post any results you get back to this forum so we can help you further.