RE: Volume, Brightness, play buttons not working on XPS 13
What is the exact system model? What is the current OS installed on the system? Are you trying to use the fn key + the f1-f12 keys?
It seems that the fn key behavior might have changed. See this link to change the behavior - http://bit.ly/2a8GiP8 - one of the listed steps should help fix the issue.
Also, please click my DELL-username and write me a private conversation with the service tag and your contact details(Name and Email) for case records.
Let us know if you have any other queries.