Hi If you want to disable speaker permanently then you just have to unplug the cable following the instrustions as given in the document goven below for
HPZ1 Workstation - Removing and Replacing the Speakers
http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c03356508
But if it is a issue with actual speaker that is faulty please call Hp customer support to have it replaced for one time fix.