Hi aircon,
Welcome to the HP Forums!
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I understand you purchased a HP Envy with a 27 inch monitor. It worked for 24 hrs but now you do not have sound. You have checked and your system has not been muted.
Here is a link to No Sound from Speakers (Windows 8).
Have you run The HP Support Assistant to aid with HP updates and resolving issues.
If you are running Windows 8, you have a built in troubleshooter.
Press the Windows key + C to open charms
Type troubleshoot sound in the search bar and press enter.
Follow the on screen prompts.
If this has not helped, I need some additional information.
How is the monitor connected?
What exact model of HP Envy beats notebook do you have? How Do I Find My Model Number or Product Number?
What Operating System are you running and is it 32 or 64 bit?
Is it the sound from the display or the computer that is not working? Are both not working?
Was there any changes from when it worked to now?
I hope this has helped.