I reviewed the specs of the HP LD4235 41.92-inch LED Digital Signage Display and it seems your display doesn't include speakers.
It is necessary to add external speakers.
http://h20195.www2.hp.com/v2/GetDocument.aspx?docname=c04238330&doctype=quickspecs&doclang=EN_US&sea... HP LD4235 41.92-inch LED Digital Signage Display (QuickSpecs/c04238330.pdf)
It is necessary to use the Audio outputs of your Display. By default the Speakers are off.
Review page 31 of the User Guide of your device. It talks about AUDIO settings.