Sometimes when I put my headphones to my laptop I couldn’t hear any sound. Since there were no sound problems without the headphones I had no idea what was causing it.
Then in ubuntu forums people were saying after playing with sound settings and reboot suddenly it starts working again. Especially when you shutdown and reboot.
The problem seems to happen in dual-boot laptops windoze/ubuntu. So if you switch from windows with a restart to ubuntu chances are there will be no sound in headphones. Then you shutdown and boot to ubuntu directly and woila there is sound.
I guess the cause of this is a shared resource, in this case the sound card and when you shutdown your laptop its freed.
This isn’t actually new for me I couldn’t mount windows drives when I switched from windoze to ubuntu, so I guess its the same with the sound card.