Make sure to use the dedicated headphone port. It's on the front of the tower. See the manual for a picture.
Earbuds  – I normally use external speakers for my Dell XPS 8920.  I tried inexpensive earbuds.  I adjusted the volume via the icon in the notification area for 100%.  The volume level did not seem very loud. Is that normal? Thanks, Jerry  
XPS 8920, earbud volume is low