I have a 13 mid-2012 MacBook Air running Mountain Lion. I recently noticed this problem with headphones. If I plug them in when the laptop is already running, they dont work at all. Sound continues to play through the speakers, and the OS doesnt recognize that headphones were plugged in. If I go to Sound on the System Preferences, the headphones dont show up at all on the list of output devices; only the integrated speakers show up. If I alt-click on the sound icon on the top bar, there are no headphones listed under output devices either. The sound continues to play normally through the speakers as if the headphones werent plugged in. However, if I plug in the headphones when the machine is turned off, and then turn it on, they work perfectly (well, sound quality isnt great, but they work). If I then unplug the headphones and plug them back in, they dont work anymore as described above. I tested this with multiple headphones, all of which work normally on other devices like my PC, iPod and Android phone. I tried resetting PRAM and that other thing (SNC?), didnt work either. I dont think its a hardware problem, since it works normally under specific conditions. Sounds more like a software bug, but Im no expert.. I searched around for issues like this, but only found generic answers which didnt solve the problem, as well as unanswered posts.