I have an HP Compaq nc8230 that has an annoying little problem with it's audio devices in Ubuntu 9.1. When I have the headphones plugged in, the audio continues to play simultaneously through the speakers no matter what. I'm not sure what kind of sound card this computer has, but in WIndows XP, Vista, 7 and OSx86 I didn't have this problem. I've tried using 3 different headphones and it didn't change anything. Please help.
Take a look at this DebuggingSoundProblems/KarmicCaveats . you can see different ways of diagnosing sound problems at Karmic.