As a tech moderator on a general computer help forum, first I confirm Jimco as correct and that the most likely fault being the common return or ground pin from the speakers to the motherboard, or jack daughter board being the source of the fault, not been inside a HP so don't know exactly where the speaker wires go to from the speakers.
Even though the machine is under warranty and should go back for repair, if that is to be avoided, then a careful dismantle and visible check will show if the plug has come off the board.
For general laptop dismantling and repairs advice have a look here--

http://www.laptoprepair101.com/