I've recently purchased an HP Envy 17 (HP ENVY 17-j053ea, Window 8 64bit) and it appears that the sounds coming out of the headphone jack is in mono not stereo unless I downgrade the sound driver.
The machine originally comes with the IDT High Definition Audio - 6.10.6454.0 driver installed, and the sound from the built in speakers sounds ok, but if plug in headphones and try the "Test" option or the "Configure Speakers" option on the devce (which are supposed to play the rising and falling tones in left and right channels) - I only hear mono through the headphones and it's slightly distorted too (quantization noise on quiet segments, before zeroing out).
If I Roll Back the Driver it gives me High Definition Audio Device 6.2.9200.16497, and when I re-run the tests the headphone is in stereo and the sound quality through headphones is good. Note that this driver doesn't seem to drive all the built in speakers, I think it's only drives the main pair, not the front pair or the sub-woofer, so using this driver isn't a long term solution. Note also that this driver doesn't support the Beats Audio control panel, i.e. none of the Beats options are available which isn't cool.
The above tests show the HW (and my headphones) are ok, but the driver isn't quite right.....
Note also that I've already encountered "Windows Update Hang at 15%" bug which I believe is audio driver related, but I've fixed this and it didn't help. I've also had a blue screen, apart from that the laptop is fine :-)