I've got a problem with my Dell Inspiron 14z, originally sold with Windows 7, upgraded to Windows 8. I've got any sound in headphones only after some boots. I've just had to reboot five times before it appeared.
Speakers are muted correctly when I plug headphones in, some Dell crapware also correctly says they are plugged in, I have no problems in dual-booted Ubuntu, so it must be a software issue.
I've tried downloading drivers from Dell website, uninstalling those I have and letting Windows install its default, but it didn't help. I haven't found anything interesting in Event Viewer.
I didn't use Windows 7 long enough on this computer to say whether the problem occurred before the upgrade.