I have an Envy X2 also and the sound is horrible. I tried to follow the instructions above and I too do not have the Enhancements tab so I can't get to the Loudness Equalization option.... are there other options to improve the quality on this machine?
These don't work for me
Right click the speaker icon in the Taskbar and select 'Playback Devices'.
Left click the default device once to highlight it ( it's usually 'speakers & headphones' ) then click the Properties button. Click the Enhancements tab and put a tick in the box next to 'Loudness Equalization'.
Click Apply to save the change and then click OK in all remaining windows and see if this has helped at all.