I have a HP ENVY 15 Notebook PC (LW897AV), running Windows 7 Home Premium 64-bit Service Pack 1. It is only a little over a year old as I got it in June 2012, so is still under warranty. Obviously I'd rather not send it back in if I don't have to.
The day before yesterday I was having no issues, as all normal sounds were present (i.e. Windows start-up/shut-down, as well as able to hear mp3 music played on Media Player). Yesterday when I started up all was quiet. I could not hear any system sounds, nor manually force any sounds to be heard. When I look at the volume mixer, I can see the green bar rising and falling (when playing music) so it seems like the sound is being generated but just not being relayed to the internal speakers and/or headphone jacks. Likewise when I open the "Sound" window, my default device is the "Speakers and Headphones, IDT High Definition Audio CODEC", and it shows the same activity on the green verticle bars. I do not use any external speakers.
The audio troubleshooter did not identify any issues. All drivers/etc appear to be running properly with no problems noted. I have not installed any new programs in the recent past (but I honestly don't know if some update occured without my knowledge).
Something similar to this happened to me a couple months ago. Sound randomly disappeared that time too, but came back just as randomly several days later. Now that it has happened again, I would like to not only fix it but determine the cause so it doesn't re-appear again and again in the future.
Any advise or suggestions would be greatly appreciated. And if there is any further information I should provide to give a better view of the situation, just let me know and I'll do my best. Thanks!