Hello jono21,
Welcome to the HP Forums, I hope you enjoy your experience!
I understand you are looking for the Windows 8.1 Audio Drivers for your HP ENVY TouchSmart 15-j051sa Notebook PC. I would like to assist you with that. I am providing you with a link to two Audio Drivers for that computer running Windows 8.1. Give them a try and it should resolve your HDMI Audio issue.
(1) IDT High-Definition (HD) Audio Driver - sp63407.exe
(2) IDT High-Definition (HD) Audio Driver - sp63555.exe
I believe this should resolve your Audio issue. Thank you for posting on the HP Forums. Have a great day!