Laptop wakes from sleep, once, due to audio controller (Windows 7)

You may want to consider updating your BIOS; I've seen other issues around the web where people have had similar "wake-once" issues like yours, and some of the posts indicated a BIOS update fixed it.

Dell has a BIOS update for your laptop that was released on March 27, so unless you got your computer less than two months ago it's quite likely you'll need this (and even if you did get it more recently, it wouldn't be a bad idea to try it).

http://www.dell.com/support/drivers/us/en/19/DriverDetails/DriverFileFormats?DriverId=7TT65&FileId=2910945453&productCode=xps-15z&urlProductCode=False

An audio driver update might not be a bad idea either; however, it seems Dell has only released one, so you may already be running this version.

http://www.dell.com/support/drivers/us/en/19/DriverDetails/DriverFileFormats?DriverId=0T0GT&FileId=2731103525&productCode=xps-15z&urlProductCode=False

This is not a direct answer but a temporary workaround while you figure out a permanent solution:

1. Write a batch file to disable/enable(toggle) audio driver.
2. Using task scheduler, schedule this batch to run on computer sleep and wakeup events(get exact event IDs from event viewer).