Putting this in a new topic as I know there are others out there wondering about the inactive headphone option in "Sounds" settings, and maybe this will be found easier than buried in the other Win8 thread.
It is a simple case of installing drivers.
After fixing this, I went back to correctly list the procedure, but the headphone option had completely disappeared, so this is from memory.
Right click the speaker icon in the system tray and select "sounds".
Then select the "playback" tab in the pop up window.
This is where the inactive headphone option should be.
Highlight it, and select "properties", and then select "properties" again.
Select "Update Driver" and then choose to search your computer, or it could say a specific location. I'm doing this from memory remember, and I'm on my XP system now.
For a reason that totally escapes me, this option defaults to a search of "My Documents", which is the last place you would look for drivers.
"Browse" to "Windows". You needn't know exactly where the relevant drivers are stored, but they are in "Windows".
Select OK, and after a search it should install the needed drivers, but don't have your headphones right over your ears when you try it, as it's set quite loud.
If anyone does this, can you note the procedure for the exact terminology at each stage.
Anyways, this worked for me, and it installed my "Realtek High Definition Audio" drivers.
Hope it works for you.